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Environmental factors in declining

human fertility

Niels E. Skakkebæk

1,2,3

✉

, Rune Lindahl-Jacobsen

, Hagai Levine

Anna-Maria Andersson

1,2

, Niels Jørgensen

1,2

, Katharina M. Main

1,2,3

Øjvind Lidegaard

3,6

, Lærke Priskorn

1,2

, Stine A. Holmboe

1,2

, Elvira V. Bräuner

Kristian Almstrup

1,2

, Luiz R. Franca

, Ariana Znaor

, Andreas Kortenkamp

Roger J. Hart

10,11

and Anders Juul

1,2,3

1,2

Abstract | A severe decline in child births has occurred over the past half century, which will lead

to considerable population declines, particularly in industrialized regions. A crucial question is

whether this decline can be explained by economic and behavioural factors alone, as suggested

by demographic reports, or to what degree biological factors are also involved. Here, we discuss

data suggesting that human reproductive health is deteriorating in industrialized regions.

Widespread infertility and the need for assisted reproduction due to poor semen quality and/or

oocyte failure are now major health issues. Other indicators of declining reproductive health

include a worldwide increasing incidence in testicular cancer among young men and alterations

in twinning frequency. There is also evidence of a parallel decline in rates of legal abortions,

revealing a deterioration in total conception rates. Subtle alterations in fertility rates were

already visible around 1900, and most industrialized regions now have rates below levels required

to sustain their populations. We hypothesize that these reproductive health problems are par-

tially linked to increasing human exposures to chemicals originating directly or indirectly from

fossil fuels. If the current infertility epidemic is indeed linked to such exposures, decisive regula-

tory action underpinned by unconventional, interdisciplinary research collaborations will be

needed to reverse the trends.

Are human populations living in industrialized regions at

risk of a catastrophic decline? With anthropogenic (that

is, caused by humans or their activities, such as emission

of greenhouse gases) climate change firmly placed on the

global agenda, there is increasing concern that human

populations (alongside those of many other species) are

at risk, unless drastic adjustments are implemented to

ensure more sustainable living. What is less evident on

sustainable development agendas, however, is that more

than half of all humans presently live in areas of the world

where birth rates have persistently declined below the lev-

els necessary to reproduce and sustain their populations

(Fig. 1)

. Transnational migration has historically had a role

in the ebb and flow of population change, and rising life

expectancy has tempered population declines in many

places

. However, with birth rates having dropped below

one per woman in some East Asian countries/regions

, it

is imperative that we understand why, how and with what

consequences ongoing fertility declines are taking place.

Of note, the word ‘fertility’ has two meanings in mod-

ern literature. Although it can sometimes be confusing,

the proper meaning is usually evident from the context

in which the word is being used. In demography, fertil-

ity is defined as the number of children (for example,

low fertility equals low fertility rates) and in biology,

fertility is defined as fecundity, or the ability to repro-

duce. In addition, the term fertility rate is often used

synonymously with total fertility rate (TFR; the average

number of live births a woman would have by the age

of 50 years if she were subject throughout her life to the

age-specific fertility rates observed in a given year; its

calculation assumes that there is no mortality) or the

general fertility rate per 1,000 persons (that is, the num-

ber of births in a year divided by the number of women

aged 15–44 years times 1,000).

The underlying causes of the current unsustaina-

ble fertility rates are unclear; however, demographic

research has provided some evidence of socioeconomic

causes

, which have been investigated in two large inter-

national studies

1,4

. A crucial and unanswered question

is, however, whether fecundity (the biological ability to

conceive) is indeed constant (as generally indicated in

✉

e-mail:

Niels.Erik.Skakkebaek@

regionh.dk

https://doi.org/10.1038/

s41574-021-00598-8

Nature reviews

Endocrinology

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Key points

• Industrialized regions have birth rates so low that their populations cannot be

sustained; declines in birth rates are generally ascribed to socioeconomic and cultural

factors, although human infertility is widespread.

• Decreasing fertility rates were already recorded around 1900 in Denmark, a few

decades after the beginning of utilization of fossil fuels that were, and still are, drivers

of modern industrialization and wealth.

• We hypothesize that declines in fertility rates might be linked to exposures to

chemicals originating from fossil fuels causing human reproductive problems and

cancer; early gestation might be a sensitive period.

• The current unsustainable birth rates will eventually result in decreasing populations.

• A key research challenge remains: how to distinguish biological from socioeconomic

and behavioural factors?

demographic publications) or whether modern lifestyles

have resulted in changes in human reproductive phys-

iology resulting in societies with a greater number of

infertile, or even sterile, couples than previously.

Here, we discuss trends in human reproductive

behaviour and health that are associated with infertil-

ity (that is, failure to establish a clinical pregnancy after

12 months of regular, unprotected sexual intercourse

including impaired semen quality, rising incidence of

testicular cancer, delays in couples’ pregnancy planning

and trends in assisted reproduction. The fact that the

changes have occurred over a period of only a couple

of generations suggests that environmental factors

have a role. We link these trends to modern lifestyles in

industrialized regions. Reviewing existing evidence, we

find support for the idea that some of the unfavourable

reproductive trends started more than 100 years ago.

contraception was introduced in Denmark in the 1960s

A similar pattern was seen in Sweden

. It is note-

worthy that in other parts of the world (Supplementary

Fig. 1), where the onset of industrialization and

economic upturn started much later than in many Euro-

pean countries/regions (for example, in many South

American countries/regions), the observed fertility

decline appeared within the past five decades.

In the countries/regions with early industrializa-

tion (that is, starting in the 1800s), a decline in fertility

rates occurred through the 1900s, although interrup-

tions occurred during world crises, including periods

of war and economic depression

6–8

. These trends have

resulted in marked demographic changes. In some parts

of the world, including Japan (Supplementary Fig. 2)

and Germany, the number of children and adolescents

has declined by 50% since the 1960s

. During the same

period, life expectancy has markedly increased in these

and other places

. As a result, there are now considerably

fewer young people and relatively more elderly people in

industrialized regions than previously, creating so-called

ageing societies. In the future, these demographic trends

will undoubtedly result in decreasing populations in

many countries/regions

. However, there is a consider-

able time lag. In Japan, where unsustainable fertility rates

were observed as early as 1961, the population size did

not peak until 2009, when deaths eventually exceeded

births in the transformed and aged Japanese population

(Supplementary Fig. 2)

Fertility rates

A well-documented, although unexplained, pronounced

decline in fertility rates began in Europe, includ-

ing Denmark, around the year 1900

(reFs

6–8

) (Fig. 2)

However, as depicted in

Fig. 2

, both world wars (World

War I and World War II) interrupted the decline. In

Denmark, whereas the impact of World War I was short,

the increase in fertility rates that started during World

War II levelled off and then persisted until modern

Author addresses

Department of Growth and Reproduction, Copenhagen University Hospital —

Rigshospitalet, Copenhagen, Denmark.

International Center for Research and Research Training in Endocrine Disruption of

Male Reproduction and Child Health (EDMaRC), Rigshospitalet, University of

Copenhagen, Copenhagen, Denmark.

Department of Clinical Medicine, University of Copenhagen, Copenhagen, Denmark.

Department of Public Health, University of Southern Denmark, Odense, Denmark.

School of Public Health, Hadassah Medical Center, Faculty of Medicine, Hebrew

University of Jerusalem, Jerusalem, Israel.

Department of Gynecology, Rigshospitalet, University of Copenhagen, Copenhagen,

Denmark.

Department of Morphology, Federal University of Minas Gerais (UFMG), Belo Horizonte,

Brazil.

Cancer Surveillance Branch, International Agency for Research on Cancer, Lyon, France.

Division of Environmental Sciences, Brunel University London, Uxbridge, UK.

Division of Obstetrics and Gynaecology, University of Western Australia, Perth, Western

Australia, Australia.

Fertility Specialists of Western Australia, Bethesda Hospital, Claremont, Western

Australia, Australia.

Lifestyles and environmental exposures

Industrial development of a society is associated with

fundamental changes in daily life, including altered

work processes and new lifestyles, which are often asso-

ciated with increased sedentary behaviour and weight

gain, in addition to the increasing risk of exposure to

industrial toxins

. Importantly, increased use of fossil

fuels, which has historically been closely associated with

industrialization of a society, accelerated with industri-

alization in the late 1800s, when improved standards

of living became possible for many people due to the

increased use of fossil fuels for home heating and

transport

15,16

. New environmental exposure patterns

also occurred. Initially, these exposures were often in

the form of the smog that is well known from London

and Los Angeles in the 1900s

. Currently, smog has

also been seen in cities with high economic growth in

the past few years (for example, in numerous Chinese

cities

). In addition, industrialization has profoundly

changed habits of consumption, including diet, clothing

and travel

. These changes in human life have globally

improved living standards and made daily life more

comfortable for many people; however, they have also

resulted in increased exposure to new synthetic chem-

icals

, all originating — directly or indirectly — from

fossil fuels

A crucial question is whether the changes in lifestyle

and environment that are associated with industrializa-

tion are causing changes to the reproductive physiology

of humans that are so extensive that a tipping point

has

been reached where sustainable human reproduction is

threatened.

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Factors that influence birth rates

Contraception, unplanned pregnancies and abortions

The availability of effective contraception, and espe-

cially the introduction of the contraceptive pill in the

1960s

, has greatly facilitated family planning and has

also had a role in improved educational attainments of

women (that is, girls and women are staying in educa-

tion for longer than previously), which is clearly linked

to reduced fertility rates

. However, the decline in the

number of births per woman had already begun around

1900, half a century before the introduction of the con-

traceptive pill

. In fact, the Danish fertility rate was

already as low as two children per woman in the 1930s

(Fig. 2)

. Despite the availability of effective contraceptive

methods, approximately half of all pregnancies in the

USA were unplanned during the period 2008–2011,

whilst in the early 1980s almost 60% of all pregnancies

in the USA were unplanned

. Although these num-

bers are most probably overestimates by up to 6% due

to the method of measurement used

, it remains to be

elucidated whether the declining rate of unintended

pregnancies in the USA was due to more careful use of

contraception than in the 1980s.

A high proportion of unplanned pregnancies are

continued to term, although a fraction result in induced

abortion

. It is noteworthy that the declining birth rate is

not due to an increased number of induced abortions, as

in most industrialized regions with declining birth rates

the curves for abortions have also been declining

25,26

Our own studies show that the decline in the ‘natural

5.5

4.5

conception rate’ (births plus induced abortions, minus

births after medically assisted reproduction (MAR))

among women in Denmark is even more pronounced

than the decline in fertility rate

27,28

A comparative analysis of pregnancy data from

Scandinavian countries/regions for the period 1975–

2013 showed stable and similar delivery rates of around

60 per 1,000 women aged 15–44 years and fertility rates

between 1.5 and 2.0 per woman

. However, during the

same period, the frequency of induced abortions was

clearly declining in Denmark and Finland, slightly

decreasing in Norway, but increasing in Sweden. During

the most recent 6-year period in the study, delivery and

induced abortion rates were compared with the rate

of hormonal contraceptive use. The latter was neither

consistently associated with birth rates nor with induced

abortion rates. Denmark, Sweden and Finland had the

highest percentage use of hormonal contraception,

the highest and lowest rate of induced abortions were

seen in Sweden and Finland, respectively, and all

countries/regions had similar rates of delivery. Thus, it

seems that factors other than the use of hormonal con-

traception must have influenced the rather different

induced abortion rates in these countries/regions.

Spontaneous pregnancy loss

A historical follow-up study from Denmark published

in 2020 that assessed all recorded pregnancy losses in

the country over a 40-year study period demonstrated

that 23% of women in Denmark had been referred with

a pregnancy loss at least once before the age of 45 years

(18% had one loss, 4% two losses and about 1% had three

or more losses)

. Notably, these numbers do not include

losses never diagnosed at a hospital such as early losses,

which are often not detected by the women themselves.

The frequency of recorded pregnancy losses in Denmark

increased from 7.5% in 1978–1979 to a peak at 10.7%

in 2000, followed by a reduction to 9.1% in 2015–2017.

What seems to have been a decrease since the year 2000

is probably just a reflection of changed clinical practice

since 2000, when routine surgical evacuation of mis-

carriages ceased. Since this change, some women who

have had a spontaneous miscarriage have not been

referred to hospital, and therefore the miscarriage was

not registered

. Data also suggest that unintended preg-

nancy loss has become more common in the USA with

a relative increase of 1–2% per year since 1990

(reF.

)

Twinning rates

It has been hypothesized that spontaneous dizygo-

tic twinning rates can be considered a proxy meas-

ure of combined high male and high female fertility

as it reflects the frequency of double ovulation and

fertilization

32,33

. The twinning rate (the proportion of

twin deliveries in relation to the total number of deliv-

eries, expressed per 1,000 deliveries) has varied signif-

icantly during the past 100 years. After 1930–1950 the

proportion of twin deliveries declined to its lowest point

in the 1970s, which is the period before the surge in

assisted reproductive techniques (ART), including

in vitro fertilization (IVF) and intracytoplasmic sperm

injection (ICSI), which are known factors favouring

European Union

Japan

USA

World

Replacement level

Total fertility rate (per woman)

3.5

2.5

1.5

Fig. 1 |

Total fertility rates in the European Union, Japan and the USA, 1960–2018.

The dashed line represents a fertility rate of 2.1, below which a population cannot be

sustained (total fertility rate is the average number of children per woman). Despite

higher birth rates in non-industrialized parts of the world, even the total fertility rate of

the total world population seems to be declining towards 2.1. Additional information on

trends in fertility rates in 43 countries across North America, South America, Europe, Asia

and Africa, 1960–2019, is shown in Supplementary Figure 1. Data from

Databank, World

Development Indicators;

Country: European Union, Japan, USA; Series: Fertility rate,

total (births per woman); time: 1960–2018.

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4.5

Total fertility rate (per woman)

3.5

2.5

1.5

0.5

00 05 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 00 05 10 15 20

19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 20 20 20 20 20

Fig. 2 |

Total fertility rate, denmark, 1901–2019.

The dashed line represents a fertility

rate of 2.1 children per woman (below which a population cannot be sustained). Note

that a downwards trend in total fertility rate started long before the introduction of the

contraceptive pill in the 1960s. The trend was interrupted by World War I and World War II.

The grey columns mark births conceived during World War I and World War II. Data from

Statistics Denmark.

twinning due to the transfer of several fertilized eggs

In fact, a significant proportion of twin deliveries (up

to 73% of all twins since the introduction of ART in the

1980s

34,35

) might be due to ART.

When considering twinning rates before the intro-

duction of ART, decreasing trends in twinning rates

were apparently paralleled with a decrease in TFR in the

same places in the world before 1980

(reFs

8,34

)

(

Fig. 2

;

Supplementary Fig. 1). Other factors might have con-

tributed to the decline in twinning rates; for example

maternal age, an important determinant of twinning

rates as increased maternal age might be associated with

an increase in twinning rate. Furthermore, a history of

oral contraceptive use has been suggested to contribute

by directly reducing the probability of double ovulation

and fertilization

A Danish study found that the dizygotic twinning

rates, adjusted for maternal age and parity, in Denmark

in the period 1931–1965 declined by 29% compared

with an overall total twinning rate decline of 22% in the

same period

. The same group also found that rates sta-

bilized in the period 1977–1981

(reF.

)

, in accordance

with observed trends in most other countries/regions.

Couple infertility and MAR

Human infertility is common in industrialized regions,

as indicated by the increasing use of MAR

. The number

of children born after MAR (IVF, ICSI, egg donation and

insemination with donor sperm or partner sperm) has

increased dramatically worldwide and the total num-

ber has now reached millions

40,41

. In Denmark, as many

as 10% of all newborn babies are now conceived after

MAR

39,42

. The need for MAR, including ART treatments,

can be due to both female and male infertility, and often

a combination of problems in both the female and the

male partner is diagnosed

Pregnancy planning

One of the major factors determining fertility rates

across the industrialized world during the past 50 years

has undoubtedly been the ability of women to have more

control over their reproductive choices. With the advent

of the oral contraceptive pill in the 1960s, millions of

women have been able to reclaim autonomy over the

timing of conception. Indeed, there was the intention,

through donations and national commitments, that there

should be 120 million new users of the oral contraceptive

pill by 2020 (Family

Planning 2020),

which underpins

the importance of the oral contraceptive pill for modern

family planning.

The consequent risk from postponing family initia-

tion is that the family size might ultimately be smaller

than in previous generations due to the age-related

decrease in fecundity. This change might be due to an

intentional decision, but for others it might mean that

they will never be able to reach their hoped for fam-

ily size due to the limitations of the female reproduc-

tive window. However, it seems that many, but not all,

women have insight into this fact

. Modelling has been

undertaken to determine a woman’s chance of having

two children based on the age she starts trying to con-

ceive, without and with the use of IVF if it is required

This study showed that if a woman starts attempting to

conceive at 37 years old, she would have a 60% chance

of achieving her goal of two children; however, if she

waited until she was 40 years old she would only have a

30% chance. If she aspired to having three children, and

was prepared to use IVF technology if appropriate for

her, to give her a 90% chance of having three children,

she should start attempting to conceive at 28 years of age,

which is considerably younger than the age the majority

of women in developed nations start their family.

When couples postpone starting a family, the aver-

age paternal age will also increase, and semen quality,

particularly motility of sperm, diminishes

45,46

. However,

the fecundity decline is much more subtle in men than in

women, as testicular function, including sperm produc-

tion, is often maintained throughout life

. By contrast,

as a woman ages, in line with a reduction in the number

of ovarian follicles, there is a progressive reduction in

normal ovulatory frequency

The often-cited main cause of a woman’s reduced

fecundity in her late 30s is poor egg quality. This

catch-all term encapsulates the increased predisposi-

tion of the oocyte to aneuploidy, as a result of chiasmata

proximal to the telomere becoming more susceptible to

mis-segregation, which is attributable to an age-related

progressive loss of cohesion proteins

49,50

. Additionally,

with a progressive reduction in mitochondrial function,

less ATP is available for spindle formation, microtubu-

lar activity and polar body extrusion, which promotes

aneuploidy

. Furthermore, it is believed that the oocyte

is at greater risk of epigenetic modification as a woman

ages

. The increased risk of aneuploidy not only leads

to a reduction in fecundity, but also to an increased

predisposition to miscarriage, which understandably

causes distress and might further delay attempts at

conception

. Oocyte quality starts to rapidly decline

for women beyond 37 years of age, which is reflected in

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the stark reduction in the success rates of IVF treatment

(Supplementary Fig. 3)

51,52

Thus, the delay in pregnancy intention is assumed

to be responsible for many cases of female infertility.

By contrast, solid data on age-specific fertility rates in

Denmark since 1901 suggest that more women in their

late 30s and 40s were able to carry pregnancies to term

in 1901 than now, even without the availability of ART

(Supplementary Fig. 4). This finding could indicate that

more couples nowadays might become subfertile with

age than previously. Polycystic ovary syndrome (PCOS),

which affects up to one in five women of reproductive

age, is the most common female disorder leading to

infertility

. However, there is no confirmed evidence

that the incidence of PCOS or other diseases affecting

the female reproductive system that lead to infertility

are rising

. For instance, it is unclear whether the rate

of endometriosis is increasing, as a result of the inher-

ent difficulty of its diagnosis, which relies on a diag-

nostic laparoscopy

. By contrast, solid evidence shows

increases in the incidence of male reproductive disor-

ders, including testicular cancer and poor semen quality

(see subsequent sections). Nevertheless, studies on the

aetiology of changing human fecundity should take both

female and male factors into account.

Since the 1990s, there has been an increased research

focus on the possible link between changing exposures

to multiple endocrine-disrupting chemicals and human

reproductive health. Such studies are demanding, as

numerous chemicals might be involved. In addition,

animal data indicate that the female and the male part-

ner might react differently to the same exposures

Therefore, the importance of couple fecundity has been

highlighted in prospective cohort designs, where eligible

couples were followed after discontinuing contraception

to try for pregnancy

. Alterations in couple fecundity

were seen according to differences in urine or plasma

concentrations of several endocrine-disrupting chem-

icals in both men and women, after adjusting for the

partner’s concentrations

56–58

. Importantly, the findings

underscore the importance of a couple-based cohort

design, which was used in a cluster of studies

59–61

sections, there are data demonstrating that the function

of the human testis is inferior compared with that of

other mammals.

Poor human spermatogenesis

Sperm count is linked to the quality of spermatogene-

sis in the seminiferous tubules

. Compared with other

mammalian species belonging to different orders,

human spermatogenesis is uniquely poor

(Table 1)

Small animals typically allocate a greater proportion

of body mass and energy to maintenance of the tes-

tes and sperm production than larger animals, which

is also related to sexual behaviour and reproductive

strategies

. Therefore, in general, species that have

the largest gonadosomatic index (testis mass divided

by body weight) also have the highest sperm produc-

tion per gram of testis (known as spermatogenic effi-

ciency; daily sperm production per gram of testis). For

instance, as shown in

Table 1

, the gonadosomatic index

in humans is approximately four to ten times lower than

that in the black-tufted marmoset (Callithrix

penicillata)

and laboratory rodents

68,70

. In addition, the seminifer-

ous tubules in humans occupy a lower percentage of the

testicular volume than is seen in the laboratory animals

included in the study

. Furthermore, the human testis

capsule occupies a much higher percentage of the testis

(~20%) than in rodents, such as mice (~4%), meaning

that in humans there is much less volume devoted to

testicular parenchyma

. Another important compari-

son is the Sertoli cell volume density. Although humans

have a high number of Sertoli cells per gram of testis,

these important somatic cells of the seminiferous epithe-

lium display a very low capacity for germ cell support

(Table 1)

. In humans, Sertoli cells occupy a high pro-

portion of the seminiferous epithelium (~40%), in con-

trast to mice in which this figure is much lower (~15%);

thus, human testes have a reduced volume of germ cells.

Therefore, not surprisingly, Sertoli cell volume density

within the seminiferous tubule is inversely correlated

with the efficiency of sperm production

The number of differentiated spermatogonial gener-

ations is another key parameter related to the magnitude

of sperm production and is used in comparative studies

among different species. In particular, the number of

spermatogonial generations dictates the number of germ

cells that enter meiosis, being characteristic of each spe-

cies and phylogenetically determined. In humans, only

two generations are observed (type A (dark and pale)

and type B), meaning that only four germ cell divisions

occur (two mitotic and two meiotic) before spermatids

are formed, whereas eight exponential divisions take

place in laboratory rodents and farm animals, such as

bulls (Bos

taurus)

and boars (Sus

scrofa domesticus)

(Table 1)

. Moreover, in humans, in addition to apoptosis

that normally occurs during the spermatogonial phase,

germ cell apoptosis during meiosis results in almost

70% loss of spermatocytes

. This loss means that the

overall rate of spermatogenesis in humans is sixfold

and 30-fold lower than that observed in the marmoset

(C.

penicillata)

and rats, respectively. Taking into con-

sideration the spermatogonial kinetics, the meiotic divi-

sions and germ cell loss during spermatogenesis in the

Male reproductive disorders

In contrast to the scarcity of available information about

trends in female fertility issues (except trends in post-

poning pregnancy), there is a large amount of litera-

ture on adverse trends in male reproductive disorders.

An important question is whether there are genetic or

environmental factors that could explain why men are

sensitive to changes in the environment. Several stud-

ies have shown genetic variation among strains of rats

and mice in susceptibility to endocrine disruption by

chemicals

62–65

. An important finding was that CD-1 mice

and Sprague–Dawley rats, which have often been used

in laboratories for testing chemical safety, might be less

sensitive to endocrine disruption than other strains of

rodents

64–66

. Interestingly, the CD-1 mice and Sprague–

Dawley rats are also good breeders

. It has not been

examined whether men are particularly prone compared

with women to endocrine disruption due to environ-

mental exposures. However, as shown in the following

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Table 1 |

Testis function and reference semen parameters in humans and other mammalian species

Parameters

Human Marmoset

rat

(Callithrix

penicillata)

224

16.6

0.08

Mouse

225

Bull

Boar

rabbit

Testis

Testis weight (g)

Gonadosomatic index (%)

Seminiferous tubules (%)

Sertoli cells per gram of testis (×10 )

Spermatogenesis

Sertoli cell efficiency

Spermatogonial generations

Meiotic index

Overall rate of spermatogenesis

Spermatogenic cycle length (days)

Spermatogenesis total duration

(days)

Spermatogenic efficiency (×10

)

Epididymis

Epididymal sperm transit (days)

Epididymal sperm reserve (×10

)

Ejaculate

Ejaculate volume (ml)

Sperm concentration (million per

millilitre)

Sperm per ejaculate (million)

Motile sperm (%)

Morphologically normal sperm (%)

1–5

>20

>40

>50

>30

0.03

1,063

2–10

150–500 0.4–0.6

230–350

150

300–2,000 25–300

10,000

40–75

65–95

45,000

50–80

70–90

5.5

0.84

8–10

0.74

5–5.8

0.08

4–15

9–11.8

161

6.6–12.7

2.1

3.0

1.3 (68)

3.2 (80)

4.1

8.0

3.5 (13)

20 (63)

15.4

3.4 (15)

97 (62)

12.9

10–12

3.0 (25)

65 (75)

8.6–8.9

39–40

38–53

8.0

3.6 (10)

65 (75)

13.5

3.2 (20)

68 (73)

9.0

3.3 (18)

39 (69)

10.9

0.5

0.36

1.7

0.8

0.1

0.6–0.8

91–93

55–64

402

0.1

365

0.4

3.1

0.21

Unless otherwise stated the source of data for testis function and the epididymal sperm reserve data shown are from

reF.

and

reF.

;

references related to semen evaluation:

reFs

226–232

. NA, not available.

Right testis plus left testis divided by two.

Testis mass divided

by body weight.

Number of round spermatids per Sertoli cell.

Number of differentiated spermatogonial generations that are usually

type A, intermediate and type B.

Number of round spermatids per each primary spermatocyte (the numbers in parentheses show the

percentage of germ cell loss based in the theoretical yield of four).

Number of spermatids formed per differentiated initial type A

spermatogonia (the numbers in parentheses show the percentage of germ cell loss based on the theoretical yield for each species

according to the mitotic divisions or generations from initial type A spermatogonia and the two meiotic divisions).

Assuming that

spermatogenesis takes 4.5 cycles.

Daily sperm production per gram of testis.

human testis, only two spermatozoa are produced out of

ten from each initial differentiated type A spermatogo-

nia, whereas in other mammals this figure is three to

four out of ten (see the overall rate of spermatogenesis

Table 1

). Therefore, even producing 1,500 sperm with

each heartbeat, human males have the least productive

testes of all mammalian species so far investigated.

The duration of spermatogenesis in a species is deter-

mined by the length of the spermatogenic cycle, which

is under the control of the germ cell genotype

and is

an important determinant of the magnitude of sperm

production. Species with shorter durations of spermato-

genesis are usually those with a higher sperm produc-

tion. This observation is well illustrated when comparing

humans to rodents in

Table 1

The sperm transit time through the epididymis

ranges from ~5 days to 16 days

in the mammalian spe-

cies listed in

Table 1

. The frequency of semen collection

does not influence this transit, and the time required

for sperm maturation within the caput and corpus of

the epididymis ranges from 2 days to 5 days. Therefore,

only a few days are required for sperm to develop their

fertilizing potential in humans and mammals in general.

As a clear illustration of inefficient spermatogenesis in

humans, the epididymal sperm reserve is quite similar

in humans and rodents, even though humans have much

larger testes

(Table 1)

. Finally, besides having a very low

number of sperm output, human semen quality, includ-

ing sperm morphology and motility, is generally worse

than in the other mammals shown in

Table 1

. It remains

to be investigated whether low spermatogenic efficiency

makes human males more vulnerable to environmental

exposures than farm animals and laboratory rodents.

Semen quality.

In 1992, evidence of a decline in sperm

concentration over half a century in Europe and the USA

was reported

. However, the data remained controver-

sial, even after 25 years. In 2017, a systematic review and

meta-regression analysis on trends in human sperm

count was published aiming to answer the question:

have human sperm counts, as measured by sperm con-

centration and total sperm count, declined? A total of

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244 estimates of sperm concentration and total sperm

count, sampled in 1973–2011, were extracted out of

7,518 publications initially screened for meta-regression

analysis

(Fig. 3)

There was a significant decline in sperm concentra-

tions between 1973 and 2011 among ‘unselected Western’

men (−1.38 million per millilitre per year, 95% CI −2.02

to −0.74;

< 0.001) and among fertile Western men

(−0.68 million per millilitre per year, 95% CI −1.31

to −0.05;

= 0.033). Among unselected Western men,

the mean sperm concentration declined, on average,

1.4% per year with an overall decline of 52.4% between

1973 and 2011. Similarly, the decline in total sperm count

in unselected Western men was on average 1.6% per

year, with an overall decline of 59.3% within the study

period. Fewer semen studies have been published from

non-Western countries/regions. However, several stud-

ies have shown a remarkable decline in semen quality

among sperm donors in China

75,76

. It is also notewor-

thy that some Chinese investigations have linked air

pollution to changes in semen quality

In Denmark, a single-centre prospective study was

initiated in 1996 to monitor semen quality among

young men from the general population (still ongoing).

Overall, the results indicate that during over 20 years of

surveillance, semen quality in this population has been

stable but low. More than one-third of the included men

had ‘low’ semen quality, defined as having one or more

semen parameters (sperm concentration, motility or

morphology) below the WHO reference limits

. When

comparing the data from this study to historical data

of Danish men examined in an infertility clinic in the

1940s, the distribution of sperm concentration in this

study was skewed to lower levels. Men examined in the

1940s had a median sperm concentration of more than

60 million per millilitre, whereas it was only 45 million

per millilitre among the men examined around year

2000

(reF.

)

. Importantly, the method of assessing the

100

sperm concentration, based on the haemocytometer, did

not differ between the two populations.

The chances of achieving a pregnancy in a given

menstrual cycle are reduced if the sperm concentra-

tion is below approximately 40 million per millilitre

80–82

(Supplementary Fig. 5). Thus, in a population such as

that of the men examined in the 1940s, only a small pro-

portion of the men had a sperm concentration below

the level where fecundity would be affected, and semen

quality might have declined previously without any

noticeable effect on men’s fertility. But with the level

observed today, a large proportion of young men have

a sperm concentration in the suboptimal range below

the threshold of 40 million per millilitre and a longer

time to pregnancy or need for fertility treatment could

be expected

Significance of changes in reference ranges for human

semen quality.

An evaluation of data on trends in semen

quality might be hampered by the fact that normality

in medicine is often defined as the range between

the 2.5 and 97.5 percentiles in a random sample from the

general population. The most recent WHO guidelines

for analysis of semen adhere to this principle

. However,

while this is a relevant approach for the most basic

areas of human physiology (for example blood levels

of sodium and potassium), this approach is not always

appropriate in medical practice. As an example, the dis-

tribution of body mass among individuals in a popu-

lation is heavily influenced by the type of diet. Using the

2.5 and 97.5 percentiles as limits of normality in a popu-

lation with general over-nutrition might lead to ‘normal’

ranges that are too high, which from a health perspec-

tive is inappropriate. Similarly, there are historical data to

suggest that ranges of normal semen quality according

to WHO guidelines are not appropriate and should be

updated, as too many men with low fecundity prospects

might be evaluated as ‘normal’

. It is also noteworthy

that according to the current WHO guidelines

, sam-

ples with as few as 5% morphologically normal sperms

(95% abnormal) are categorized as within the ‘normal’

reference range (Supplementary Fig. 6).

Testicular cancer

As testicular cancer, which mainly occurs in young men,

is associated with disorders such as undescended testis,

decreased semen quality, infertility and childlessness, it

is pertinent to include trends in this disease in a general

discussion of trends in male reproductive health

A remarkable worldwide increase in the incidence

of testicular cancer has taken place during the past

100 years. Ever since the establishment of national can-

cer registries, first initiated in 1943 in Denmark

, a sig-

nificant worldwide increase in testicular cancer has been

observed, particularly among white men

. Although the

increasing trend was briefly interrupted in birth cohorts

of Nordic men born during the world wars

88,89

, the inci-

dence rates of testicular germ cell cancer in Denmark

doubled between 1943 and 1962

(reF.

)

. However, it is

not known when this secular trend started. Interestingly,

historical mortality data from countries/regions with

early industrialization onset, including England and

Sperm concentration (million per ml)

52.4% decline

Unselected

Western

Fertile Western

Unselected

other

Fertile other

1970

1980

1990

2000

2010

Year of sample collection

Fig. 3 |

changes in average sperm concentrations 1973–2011.

The slopes of sperm

concentration were estimated as a function of sample collection year using weighted

meta-regression models, adjusted for predetermined covariates and modification by

fertility (‘unselected by fertility’ versus ‘fertile men’) and geographic group (‘Western’,

including North America, Europe, Australia and New Zealand, and ‘other’, including Asia,

Africa and South America). Sperm concentrations declined significantly between 1973

and 2011. Figure reprinted with permission from

reF.

, OUP.

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the USA, show that the secular trend in testicular germ

cell cancer mortality started as early as around 1900

(reFs

90,91

)

Currently, there are 74,500 estimated new testicular

cancer cases per year globally

. While its contribution

to overall cancer incidence is small (1%), testicular

cancer is the most common cancer in young men aged

15–44 years in Europe, the Americas and Oceania

One-third of all cases occur in Europe, with the highest

age-standardized (world) incidence rates of >10/100,000

observed in Norway and Denmark

92,93

The secular trend is continuing, with testicular can-

cer incidence rates increasing worldwide, including in

parts of Latin America and Asia that previously had very

low incidence

94,95

(Fig. 4)

. In Europe, an attenuation of

increasing incidence has been reported from the highest

risk countries/regions, while the largest increases in rates

have been observed in lower risk countries/regions, such

as Finland, the Baltic countries/regions and some coun-

tries/regions in southern and eastern Europe

. Thus, the

overall testicular cancer burden continues to increase in

Europe, most notably in eastern Europe, with a 32% pre-

dicted increase in the number of cases between 2010 and

2035, as well as globally

In female individuals, GWAS have investigated

traits such as age at the birth of the first child and the

number of children ever born (also influenced by

social factors)

102

, self-reported age at menarche and

menopause

103,104

, and anti-Müllerian hormone levels

105

as indirect measures of the ovarian reserve. In the

study of anti-Müllerian hormone levels, only one var-

iant reached significance (rs16991615) and this variant

was also found to be associated with differences in age

at menopause

106

. The rs16991615 variant is a missense

variant located in exon 9 of

MCM8,

which is required

for homologous recombination, and the variant might

reduce repair of double strand breaks causing arrest of

follicle development, as observed in

Mcm8-knockout

mice

107

. In general, several of the GWAS have indicated

that DNA repair is critical for follicular development,

but it is questionable whether an accumulation of such

variants has occurred in the past five to ten decades and

whether the modest effect sizes observed have any role

in the observed decreasing fertility trends.

Epigenetic variation and reproduction.

While the con-

tribution of genetic factors to decreasing fertility trends

is questionable, it is likely that epigenetic patterns can

be affected in a time window encompassing three to

four generations (corresponding to the period where

decreasing fertility trends have been observed; see pre-

vious discussion). Epigenetic marks are dynamic and can

be affected by lifestyle and environmental exposures,

for example, and it is well-documented that epigenetic

marks in the germline can be inherited and cause effects

in the following generations

108

. The dynamic epigenetic

patterns, however, also make it difficult to associate such

marks with reproductive traits with certainty. The most

studied epigenetic marks include methylation of the

DNA strand at CpG sites and small RNAs.

Changes in DNA methylation in blood samples have

been associated with, for example, pubertal timing in

both boys and girls

109,110

and with the average number

of children a woman gives birth to

111

. DNA methylation

marks have also been linked to lifestyle and environmen-

tal exposures

112,113

. Other studies have shown that effects

of in utero exposure to famine

114

or maternal smoking

during pregnancy

115

can be detected in the blood methy-

lome in adult life. However, although such blood DNA

methylation marks have been shown to be a proxy for

alterations in reproductive tissues such as the testis

109

it is currently unknown if alterations in germline DNA

methylation have any effect on fertility. It is notewor-

thy that the ejaculate also contains somatic cells, which

can affect measurements of DNA methylation marks in

sperm

116

, and methylation marks are erased and repro-

grammed in the preimplantation embryo. Nevertheless,

several chemicals have been shown to be associated

with alterations in the sperm methylome

117–119

in rats, in

which changes in the sperm methylome can be observed

in the third (F3) generation after exposure to the

agricultural fungicide vinclozolin

120

Accumulating evidence indicates that small RNAs

inside sperm can carry lifestyle and exposure information

between generations

108,121

(Fig. 5)

. Although spermatozoa

that leave the testis are unable to react to environmental

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Possible biological mechanisms

Roles of genetic and epigenetic factors

To our knowledge, no single genetic or epigenetic factor

has been shown to affect fertility on a population scale.

Obviously, genetic or epigenetic factors negatively affect-

ing fertility are not likely to survive in the population,

but the increased use of MAR (as discussed already), and

especially ICSI, bypasses the natural negative selection

pressure, and enables the accumulation of genetic and

epigenetic variants with subtle effects on fertility in the

population. Hence, genetic and epigenetic variants with

even subtle effects on fertility should be of concern, if

they accumulate in the population.

Genetic variants with effects on reproduction.

men, two studies have identified variants (found near

DPY19L2

(reF.

)

and

SPATA16

(reF.

)

) associated with

globozoospermia (a rare form of infertility in which

the sperm cells are abnormal). However, genome-wide

association studies (GWAS) in men with oligospermia

(low sperm count) or azoospermia (no sperm) have been

quite inconclusive. Very few variants overlap between

the conducted studies

98,99

, which probably reflects the

fact that the phenotype (male infertility) can be caused

by a multitude of different factors. Sequencing initia-

tives have, therefore, started to identify rare mutations

causing non-obstructive azoospermia, which is a more

precisely defined phenotype

100

Mutations causing non-obstructive azoospermia

have been identified in several genes

101

, but can only

explain non-obstructive azoospermia in a small fraction

of all cases. The largest known genetic effect on male

fertility still relates to the sex chromosomes, X and Y,

including microdeletions in the AZF regions on the

Y chromosome and the presence of supernumerary

X chromosomes, as found among men with Klinefelter

syndrome

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changes by transcription, they engulf extracellular ves-

icles during transit in the epididymis. These vesicles,

called epididymosomes, are derived from somatic cells

of the epididymal epithelia and contain both proteins and

small RNAs

122

. Mouse studies have suggested that dietary

Northern Europe

changes cause alterations in the small RNA content of

the epididymosomes, which in turn can affect embryo

development

123

. Direct injection of either sperm-derived

or synthetic RNAs that are upregulated in the sperm of

obese mice into fertilized eggs resulted in offspring with

Asia and Oceania

The Americas

Age standardized incidence rate per 100,000

1.5

0.7

0.5

1980

1990

2000

2010

1980

1990

2000

2010

1980

1990

2000

2010

Year

Norway

Denmark

Ireland

Sweden

Eastern Europe

Year

Finland

Iceland

Estonia

Lithuania

Chile*

USA: White*

Ecuador*

Costa Rica

Colombia*

USA: Black*

Year

New Zealand

Australia*

Israel

Turkey*

Japan*

Western Europe

China*

Republic of Korea*

Philippines*

Thailand*

India*

Southern Europe

Age standardized incidence rate per 100,000

1.5

0.7

0.5

Slovakia

Czech Republic

Bulgaria

1980

1990

2000

Poland*

Belarus

Slovenia

Croatia

1980

1990

Italy*

Spain*

2000

2010

Switzerland*

Germany*

The Netherlands

1980

1990

2000

Austria

France*

2010

* Regional registries

Year

Fig. 4 |

Testicular cancer incidence trends in selected countries/

regions worldwide.

Note that the incidence rates are increasing globally

and are generally highest in regions with white populations of European

origin. Several subnational registries were used to produce national

estimates for the figure: Australia (NSW, South Australia, Tasmania,

Victoria, Western Australia), Chile (Valdivia), China (Shanghai, Hong Kong),

Colombia (Cali), Ecuador (Quito), France (Calvados, Doubs, Isere),

Germany (Saarland), India (Chennai), Italy (Modena, Parma, Ragusa,

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Romagna), Japan (Miyagi, Nagasaki, Osaka), Korea (Seoul), Philippines

(Manila), Poland (Kielce), Spain (Basque Country, Granada, Murcia, Navarra,

Tarragona), Switzerland (Geneva, Neuchatel, St Gall-Appenzell), Thailand

(Chiang Mai, Khon Kaen, Lampang, Songkhla), Turkey (Antalya, Izmir), UK

(England, including East of England, East Midlands, North East, North West,

London, South East, South West, West Midlands, and Yorkshire and the

Humber; Scotland), US (SEER-Black, SEER-White). Figure adapted with

permission from IARC

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Environment

and lifestyle

Epigenetic

drift

Altered small

RNA proﬁle

Epigenetic

reprogramming

aﬀected

Environment

and lifestyle

Environment

and lifestyle

Age

Generation 1

Reduced

embryo quality

Generation 2

Slightly reduced

semen quality

Generation 3

Reduced semen

quality

Fig. 5 |

illustration of epigenetic drift.

Environment-mediated and lifestyle-mediated epigenetic drift in the germline

might be passed on by small RNAs to subsequent generations. With increasing age of the individual, the sperm

epigenome is likely to acquire a range of epigenetic alterations that can be passed on to subsequent generations.

Although the concept has been established in animal models, it remains to be validated in humans.

glucose intolerance and obesity

124

. Small RNA-mediated

changes are hence able to survive and potentially affect

epigenetic reprogramming in the early embryo. Similarly,

stress-induced changes in the small RNA profile of

human sperm have been observed, albeit only in a lim-

ited number of studies

121

. In human sperm, the levels of

a microRNA, miR-191-5p, have been correlated with

both sperm morphology and embryo quality

125

. Hence,

it is likely that both lifestyle and environmental exposure

mediate changes in the sperm small RNA profile, which

might affect health and fertility in subsequent genera-

tions. Such mechanisms could potentially accumulate

across generations and theoretically have a role in the

decreasing fertility rates.

Advanced paternal age as a risk factor for genetic

abnormalities in offspring.

Concern has been raised

that increasing paternal age due to delays in pregnancy

planning could result in an increase in the number of

spontaneous mutations in sperm cells and lead to still-

births, rare syndromes, cancer and mental health disor-

ders in offspring

126,127

. Although each of such cases might

be severe, the quantitative role of these rare mutations

for reproductive health in general is minor.

Gonadal dysgenesis

It is well documented that synthetic toxins, including

pesticides, can cause problems for adult reproductive

functions when exposure occurs in adulthood; however,

these changes are mainly reversible, even in individuals

with azoospermia

128–130

. By contrast, fetal damage of the

developmental processes of the gonad can cause perma-

nent reproductive effects in adulthood, and potentially

also congenital malformations such as cryptorchidism

and hypospadias

131

. Adult consequences of fetal malde-

velopment might be testicular cancer, spermatogenic

disorders and infertility

131

It has been recognized for decades that the fetal testis

of mammals is particularly sensitive to external expo-

sures, including irradiation

132–134

, and landmark toxico-

logical research has revealed that perinatal exposure to

phthalates can cause reproductive symptoms in adult rats,

including low testosterone levels, undescended testes,

hypospadias and spermatogenic abnormalities

135,136

These symptoms seem to be related to a dysgenesis of the

fetal testis, including abnormal development of Leydig

and Sertoli cells

(Fig. 6)

. Although the effects of phtha-

lates on fetal gonads have been most extensively stud-

ied, antiandrogenic effects of several other endocrine

disrupters on the fetal testis have also been observed.

Interestingly, a masculinization programming window

has been identified

137–140

during which disruption of the

normal, endogenous sex hormone activity is irreversi-

bly detrimental for the development of the male gonads

and genitalia

140

(Fig. 6)

. Apart from phthalates, chemicals

with the ability to displace androgens from the androgen

receptor (that is, certain azole and imidazole pesticides,

bisphenols and parabens) or to inhibit steroidogenic

enzymes (that is, the pesticides prochloraz and linuron)

can also negatively affect male sexual differentiation

138

Other molecular pathways, such as a disrupted Sertoli

cell differentiation via diminished prostaglandin levels

caused by certain pain killers, or activation of the aryl

hydrocarbon receptor by polychlorinated dioxins and

biphenyls, also have a role

141,142

. There is experimental

evidence that these pathways interact to produce mixture

effects after combined exposure to these chemicals

138

(Fig. 6)

. Future comprehensive evaluations of the impact

of chemical exposure on male reproductive health will

have to consider the joint effects of phthalates, azole

and imidazole pesticides, bisphenols, parabens, analge-

sics, polychlorinated dioxins and biphenyls and other

endocrine-disrupting chemicals.

In addition, the fetal female reproductive system

might be vulnerable

143,144

, although it seems to be less

sensitive than the male reproductive system to envi-

ronmental exposures, for example, to phthalates

145

However, a group of biologists have proposed ten

putative adverse outcome pathways relevant to female

reproductive disorders to be considered in toxicological

testing as an important step towards safeguarding the

reproductive health of human females

146

Gonadal dysgenesis and testicular cancer

Most of the evidence on the reproductive effects of endo-

crine disrupters is derived from animal studies; however,

there is also some human evidence that environmen-

tal exposures are associated with the development of

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undescended testis, hypospadias, infertility and testicular

cancer

147–150

. The strongest evidence of a fetal origin of

male reproductive disorders comes from epidemiological

investigations showing birth cohort effects with regard

to the risk of developing germ cell cancer in young

adulthood

88,89

. For instance, men born during World

War II (and possibly also during World War I) turned out

to have a decreased risk of germ cell cancer, which is in

line with this theory

88,89

. During World War I and World

War II, the increasing trends in consumption of fossil

fuels were dramatically interrupted due to import restric-

tions and so the population was probably less exposed to

pollution from coal and oil and other consumer products

than previously (Supplementary Figs 7,8).

Migration studies, where populations move between

areas with different risks of testicular cancer, are also in

line with the hypothesis that testicular cancer is of fetal

origin: the country/region where an individual is born

determines the risk, even if the individual migrates as a

Example

chemicals

Azoles,

imidazoles

Active

metabolites

MIE

•

Displacement

of androgens

from the

androgen

receptor

•

Disruption

of binding

to DNA

response

elements

Inhibition of

steroidogenic

enzymes

Inhibition of

HMGCoA

reductase

???

Downregulation

of genes for

cholesterol

transport and

steroidogenesis

Decreased INSL3

synthesis

Diminished levels

of prostaglandins

in Sertoli cells

and germ cells

Disrupted

development

of gubernaculum

Sub-optimal

SOX9 activation

Incomplete

arrest of germ

cells

Disrupted

diﬀerentiation

of Sertoli cells

TCDD

(dioxin)

Activation of

the aryl

hydrocarbon

receptor

???

Suppressed

androgen

synthesis

and decreased

androgen levels

Decreased gene

expression and

protein synthesis

in androgen-

dependent

tissues

Key event 1

Key event 2

child

151–153

. Furthermore, studies of the precursor cells of

germ cell cancer (germ cell neoplasia in situ) have shown

that they have characteristics of fetal gonocytes

154

and

also express the same fetal markers as normal fetal gono-

cytes, including OCT4 and NANOG

155,156

, which are now

used as diagnostic markers for germ cell neoplasia in situ

by many pathology laboratories

157

(Fig. 7)

A hypothesis of a human testicular dysgenesis syn-

drome arose from these and other studies in the late

1990s

85,140,158

(Fig. 8)

. The hypothesis suggests that the

syndrome is caused by maldevelopment of the fetal tes-

tis resulting in one or more of the following symptoms:

germ cell cancer, low testosterone levels, undescended

testis, hypospadias and impaired spermatogenesis. An

important confirmatory observation was that several

genetic mutations involving the differentiation and mat-

uration of the male gonad can induce symptoms of tes-

ticular dysgenesis syndrome, sometimes all symptoms,

including testicular cancer

159

. However, in most patients

Key event 3

Key event 4

Adverse outcome

Prochloraz

Linuron

Disturbed

balance

between

apoptosis and

cell proliferation

in androgen-

dependent

tissues

Decrease in

anogenital

distance and

retained

nipples in

male oﬀspring

Simvastatin

•

Malformations

in reproductive

tissues

(hypospadias,

prostate

agenesis, testis

atrophy)

•

Reduced sperm

counts

•

Undescended

testes

•

Germ cell

tumours

Phthalates

(monoesters)

paracetamol,

ibuprofen

???

Undescended

testes

Paracetamol,

aspirin,

ibuprofen

COX 1, 2

inhibition

Reduced sperm

counts

Fig. 6 |

Adverse outcome pathway network for the induction of male

reproductive malformations.

Red cells depict pathways for androgen

receptor antagonism and downregulation of steroidogenic enzymes, grey

cells are for phthalate-mediated events, yellow cells are for the insulin-like

peptide 3-mediated pathway leading to cryptorchidism, and blue cells

highlight the prostaglandin-mediated pathways. Green cells are for the

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dioxin-induced pathway leading to poor sperm counts. The adverse

outcome pathway network is based primarily on observations in animal

models

142,212

. COX, cyclooxygenase; HMGCoA, hydroxymethylglutaryl-

CoA; INSL3, insulin-like peptide 3; MIE, molecular initiating event; TCDD,

2,3,7 ,8-tetrachlorodibenzo-p-dioxin. Figure adapted with permission

from

reF.

141

, Elsevier.

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50 m

testicular cancer are linked

165–168

. In Denmark, the inci-

dence (total number of new cases divided by the popu-

lation at risk) of testicular cancer among men from

the 1940s to 2010 increased by approximately 300%

and has levelled off since 2000; however, levels are still

among the highest in the world

. Also noteworthy is

that there is evidence of a significant drop in average

sperm counts among Danish and other European men

since the 1940s

. In addition, a secular decline in testo-

sterone levels has been observed, although changes in

lifestyle factors, such increased BMI, also seem to have

a role in these trends

169,170

. Taken together, there seems

to be little doubt that environmental effects are behind

the trends in testicular cancer; however, the specific

exposures remain to be determined.

GCNIS

50 m

Normal seminiferous tubule

Role of modern lifestyles

Smoking

Tobacco smoking in young men has negative effects

on their semen quality, as demonstrated in a large

meta-analysis

171

. However, several studies suggest that

maternal smoking during pregnancy is a stronger predic-

tor of poor semen quality than the men’s own smoking

in adulthood

78,172,173

. There is also evidence that female

smokers have lower fecundability than non-smokers,

and a similar trend has been seen for male smokers in

relation to fecundability

174–177

. In addition, e-cigarette use

has been linked to impaired semen quality

178

; however,

little is known about the impact of e-cigarette use in

relation to fertility outcomes. The impact of marijuana

smoking has been investigated, but with conflicting

results

178–180

Alcohol

Another prevalent lifestyle factor that is widely debated

in relation to reproduction is alcohol consumption.

A follow-up study of 430 Danish couples trying to con-

ceive showed that the odds ratio of conceiving decreased

with increasing alcohol intake among women, with no

clear pattern between alcohol intake in men and time to

pregnancy

181

. However, a meta-analysis from 2016 that

included 15 studies and more than 16,000 men found

that alcohol intake was adversely associated with semen

volume and morphology whereas no effects were seen in

relation to sperm concentration and motility

182

Diet

In some studies, adherence to healthy dietary patterns

has been positively associated with semen quality. These

diets are typically characterized by food groups such as

vegetables, fruits, nuts, whole cereals, seafood, poultry

and low-fat dairy products

183,184

. In women, reduced

intakes of fruit and increased intakes of fast food in the

pre-conception period have been associated with infer-

tility and modest increases in time to pregnancy

185

. The

odds of infertility have been reported to be two to three

times higher in US women who consumed meals not

prepared at home (including fast food and ready-to-eat

foods) than in those who did not

186

. Interestingly, both

the male and female partner’s intake of sugar-sweetened

beverages has been adversely associated with couple

fecundity (assessed as time to pregnancy)

187

www.nature.com/nrendo

50 m

Fig. 7 |

Expression of the embryonic marker ocT4 in

adult germ cell neoplasia in situ is similar to expression

in germ cells (gonocytes) in normal fetal gonads.

| Testicular tissue from an adult man with germ cell neo-

plasia in situ (GCNIS) expressing the embryonic marker

OCT4 (red in immunohistochemical reaction).

| Box out-

lined in part

at higher magnification. This image shows

GCNIS and a normal seminiferous tubule.

| For compari-

son, this image shows a healthy human fetal testis at gesta-

tional week 14 (from an induced legal abortion), with the

same immunohistochemical staining for OCT4. Note

the similarity between phenotypes of the normal gono-

cytes expressing OCT4 (red nuclei, embedded in fetal

Sertoli cells) and the GCNIS cells in part

with testicular dysgenesis syndrome symptoms, no

genetic alterations have been found and environmental

exposures are likely to be involved

131

The links between testicular cancer, undescended

testis, semen quality, decreased fertility and lower

testosterone levels exist both at the individual and

the population levels. In particular, the association

between congenital undescended testis and increased

risk of developing testicular cancer in adulthood is

quite strong, at both the individual level

160–162

and the

population level

163,164

. There is also good evidence that

poor semen quality, decreased testosterone levels and

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Obesity

Overweight is well established as a risk factor for

reduced reproductive function in both sexes, and for fer-

tility treatment in most public health services there are

restrictions regarding access based on upper BMI limits

for female individuals

188

. A review of 49 studies con-

cluded that following ART, women with overweight or

obesity had a lower live birth rate than women with a

BMI in the normal range

188

. Fecundity of men might also

be hampered by obesity

189

. Regarding semen quality, the

overall conclusion from a meta-analysis including more

than 13,000 men from the general population and men

attending fertility clinics was that there is a U-shaped

association between BMI and semen quality with both

underweight and overweight being associated with an

increased risk of oligospermia or azoospermia

190

. That

both underweight and overweight were associated with

reduced semen quality has been confirmed in a study

of almost 4,000 sperm donors with repeated semen

sampling

191

Physical activity

Interestingly, increased levels of physical activity (within

a moderate range) might, independent of obesity, be

beneficial for semen quality

192–195

. In line with these

observations, a randomized controlled trial of 419 infer-

tile men showed that patients randomized to moderate

aerobic exercise over 24 weeks had improved semen

quality

196

and numerous other modern products. According to the

US Energy Information Administration

199

, approximately

7% of fossil fuels are consumed for non-combustion use

in the USA. In China, which has become one of the main

producers of chemicals for feedstocks for industry

200

the percentage might be even higher. The environ-

mental effects of usage of fossil fuels are, therefore, not

only increasing CO

levels and effects on climate, but

also increasing exposures of humans to mixtures of

thousands of chemicals

19,201–205

There are several major sources of exposure to poten-

tially harmful chemicals, including, combustion of coal, oil

and gas products when used as energy sources in houses,

cars, trains and aeroplanes

206,207

. There is also direct con-

tamination of chemicals released from consumer pro-

ducts made from fossil fuels, such as plastics, textiles,

building materials, cars, food additives, food packaging

materials, pesticides, cosmetics and pharmaceuticals.

In addition to indirect contamination via an environ-

ment polluted with rubbish containing these materials,

where chemicals leak into the air, rivers, lakes and seas

they might eventually end up in drinking water and our

diet, including dairy products, meat and fish

. And,

finally, exposures can occur from fossil fuel production

sites (such as fracking

208

) or oil spill accidents

209

However, except for disasters with major leakage of

chemicals (such as the Seveso disaster in Italy in 1976)

210

the precise origins of chemicals present in human tis-

sues are generally not known

204,211

. Numerous potentially

harmful chemicals have been found in samples of blood,

urine, semen, placenta and breast milk of all humans

investigated

. They have also been demonstrated in

human adipose tissues, such as the persistent chemicals

dioxins, polychlorinated biphenyls (PCBs), dichlorodi-

phenyltrichloroethane (DDT) and flame retardants

Other chemicals (such as phthalates, bisphenols, per-

fluorinated compounds (PFCs), pesticides and some

UV filters), including those that originate from the use

of plastics, building materials, food, food packaging,

cosmetics, sunscreens and drinking water, belong to the

so-called non-persistent chemicals, which are excreted

from the body within hours

. It is noteworthy that many

of these chemicals have endocrine-disrupting properties

In male individuals

Reduced semen

quality

In pregnancy

Environmental factors

For example, endocrine

disrupting chemicals

Genetic defects

Disturbed Sertoli

cell function

Testicular

dysgenesis

Decreased Leydig

cell function

Androgen

insuﬃciency

Reduced hormone

production

Hypospadias

Cryptorchidism

Impaired germ cell

diﬀerentiation

GCNIS

Testicular cancer

Industrialization

The industrial revolution, which was beginning around

1800 and accelerating from the mid-1800s, was based on

the exploitation and consumption of fossil fuels, first coal

and later also oil and gas. The increase in the use of fossil

fuels was fairly modest in the 1800s

197

. However, during

the late 1900s and the 2000s the world consumption of

fossil fuels increased exponentially

197

. It is noteworthy

that fossil fuels are not only energy sources, but they

are also the basic raw materials for the production of

more than 100,000 synthetic chemicals used in modern

materials, such as plastics

198

, pesticides, pharmaceutical

products, cosmetics, furniture, clothing, cars, aeroplanes

Fig. 8 |

Testicular dysgenesis syndrome.

The hypothesis shown here links fetal maldevelopment of the male gonads

to congenital malformations visible at birth and late-onset symptoms occurring in adulthood, including GCNIS

(germ cell neoplasia in situ) that develops into germ cell cancer (seminoma and non-seminoma), and/or infertility

and/or decreased testosterone production

. Some patients with testicular dysgenesis syndrome have all symptoms,

others only one or two.

Nature reviews

Endocrinology

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and have been shown to interfere with reproduction

in non-human animals, both in laboratory settings

and wild animals, including some that are threatened

species

19,212,213

. The literature on human effects is, how-

ever, sparse. Nevertheless, an analysis of the burden

of endocrine disrupters for male reproductive health

estimated that endocrine-disrupting chemicals might

contribute substantially to male reproductive disorders

and diseases, with an associated annual cost of nearly

€15

billion in the EU alone

214

Although the development towards more fossil

fuel-based industrialization has been increasing dur-

ing the past 150 years, this trend was interrupted in

Denmark during World War I and World War II (Supple-

mentary Fig. 8). Interestingly, the ‘testicular cancer risk

curve’ for birth cohorts of men born in Denmark during

World War II mirrors the curve for import of fossil fuels

(Supplementary Fig. 8) in line with evidence that this

cancer has a fetal origin from embryonic germ cells

215

(Fig. 7)

. It is also noteworthy that the fertility rate changed

during World War I and World War II

(Fig. 2)

since the start of industrialization, a multidisciplinary

perspective is needed involving disciplines capable of

producing evidence on both the behavioural and the

biological components.

The most important measurable biological factor

acting on fertility is fecundity. Fecundity is the result

of our genes and the environment interacting with

these genes (that is, how genes are expressed in differ-

ent environments). An approach for understanding the

biological component of the declining fertility is thus

to understand how genes and environment influence

fecundity. Individual fecundity depends on several

individual phenotypes: the quality of gametogenesis,

migration of the spermatozoa to the oocyte, fertilization,

implantation and survival of the conceptus. Measures

of couples’ fecundity have most often been estimated

using a pregnancy-based approach, in which waiting

time to pregnancy (the number of months of unpro-

tected intercourse before conception) is retrospectively

assessed among couples who eventually have a proven

pregnancy

217

. A twin study found that the causal genetic

component for time to pregnancy differs by sex (6% for

men and 30% for women) but that most of the causal

variation is explained by environmental factors

218

To truly understand couples’ fecundity, it is thus

essential to focus on the environmental factors acting

throughout the lives of the individuals. Unfortunately,

time to pregnancy is a biased measure of the true fecun-

dity as it excludes couples who do not become pregnant,

thus leading to an overestimate. A better measure of true

fecundity is the prospective cohort approach, which does

not have this limitation because couples are recruited

before they start unprotected intercourse and are then

followed to monitor the occurrence of a pregnancy

219

However, studies suggest that a high proportion of preg-

nancies are unplanned

and they might not be included

in prospective time to pregnancy studies, which could

bias the results, as such couples might be more fecund

than couples planning a pregnancy

220

. Thus, study find-

ings cannot be directly extrapolated to the general pop-

ulation. Another approach is to include unsuccessful

attempts to become pregnant retrospectively. However,

the quality of recall of the occurrence and duration of

such unsuccessful attempts to become pregnant has not

been assessed and might be poor

220

. Because of the draw-

backs of the above-mentioned approaches, the so-called

current duration designs have been suggested

221

in which

retrospective information about current attempts is

obtained thereby making it possible to include unsuc-

cessful attempts and unplanned pregnancies

221

. This

method thus offers the possibility to examine the first

step for understanding the biological causal component

for our decreasing fertility, namely the fecundity in our

populations.

The inclusion of a population sample of people of

common reproductive ages (for example, women aged

18–45 years and men aged 18–55 years) is essential

for a population-based approach to assessing fecun-

dity. Within this sample, it is important to understand

the observed patterns found for the individual cou-

ple’s fecundity, thus separately examining the male and

female components. For instance, semen analysis and

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A research challenge

The difficulty in distinguishing between the biological or

behavioural factors, including educational attainment

that affect human fertility has led researchers to con-

clude that the question on the causality underlying

the decline in human fertility cannot be answered

216

The essence of the problem is that behavioural factors

(which are influenced by social and cultural factors,

for instance) can modulate the effects of biological fac-

tors causing misleading conclusions

216

. This problem

is further complicated by the fact that fertility involves

gametes from two individuals, and that the success thus

depends on both individuals. To understand the com-

plex scenario of human fertility and its deterioration

100

2.1

2.0

Percentage (%)

1.8

1.6

1.4

1.2

1.0

0.8

Generation 1

Generation 2

Generation 3

Time point 0

Fig. 9 |

levels of unsustainable fertility rates and population sizes (newborn babies)

over three generations.

A population is sustained if the total fertility rate is 2.1 (total fer-

tility rate is the average number of children per woman). Almost all industrialized regions

have rates below that level. If the current unsustainable rates persist, considerable

demographic changes will occur within one to three generations (excluding migration),

resulting in ageing societies followed by population decline, shown here as percentage

declines in the number of newborn babies.

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Box 1 |

Transdisciplinary research needed

Broad collaboration between researchers in life sciences and social sciences, including

anthropology and demography, is needed to answer important questions.

• How can we establish methods to distinguish between voluntary and non-voluntary

childlessness in populations with unsustainable reproduction?

• How can we develop new methods to distinguish between the role of male and

female factors in couple fecundity?

• Can we identify novel biomarkers in early life that predict the adult reproductive

capacity of an individual?

• What are the biological mechanisms that link testicular cancer to poor

spermatogenesis and other reproductive disorders in young men?

• Why are there multiple reports on adverse trends in male reproductive health, but not

similiar reports on female reproduction?

• Is it possible that exposures to industrial endocrine-disrupting chemicals are more

harmful for the male than the female reproductive organs, due to the anti-androgenic

and oestrogenic properties of many of these chemicals?

• Why is human spermatogenesis much poorer than spermatogenesis of most other

mammals?

• Why have serum levels of testosterone in human males declined during the past

generation? Is it due to environmental exposures or does the obesity epidemic have

a role as well?

• What can we learn from scientists studying reproduction of endangered wildlife

species?

• Is it possible that human fertility rates will return to sustainable levels in countries/

regions where they have been below sustainable levels for decades?

variation by environmental exposures, as well as analyses

of reproductive hormones and environmental chemicals,

will help identify fecundity biomarkers. Equally impor-

tant is obtaining information on the environmental

exposures (such as lifestyle and the working environ-

ment) that might influence fecundity in the couples.

The current duration design offers a direct possibility

for doing so, which has been illustrated by the fact that

tobacco smoking in women is associated with a doubling

in the median duration of unprotected intercourse before

pregnancy

221

. The feasibility of the current duration

design has further been demonstrated as information

on fecundity was obtained without significant selection

bias

222

, which suggests that this design is a strong basis

for further understanding of fertility problems. However,

to get an understanding of the fertility changes in the

population, we also need data on behavioural factors,

such as voluntary and involuntary childlessness and the

complex economic, social and educational profiles of

couples currently attempting to conceive. We also need

to describe pregnancy planners and non-planners, and

changes in planning behaviour over time. The deter-

minants controlling these behavioural factors are man-

yfold, and include changes in an individual’s identity

in connection with childbearing and raising children.

It is thus only multidisciplinary studies that can help us

understand the reasons behind the ongoing decrease in

fertility rates.

Vollset, S. E. et al. Fertility, mortality, migration, and

population scenarios for 195 countries and territories

from 2017 to 2100: a forecasting analysis for

the Global Burden of Disease Study.

Lancet

396,

1285–1306 (2020).

Lee, S. J., Li, L. & Hwang, J. Y. After 20 years of low

fertility, where are the obstetrician-gynecologists?

Obstet. Gynecol. Sci.

64,

407–418 (2021).

Conclusions

A crucial problem is that knowledge about the causes

underlying the global downturn in births is not avail-

able. The trend was already visible in Denmark around

1900, when fossil fuel-based industrialization had just

started, and occurred without the use of modern con-

traception, which was introduced half a century later. It

remains to be elucidated whether the decreasing fertility

rates are linked to changes in our biological systems due

to environmental exposures or to behavioural socioec-

onomic changes caused by modern lifestyles, or due to a

combination of both.

In support of a biological hypothesis, the trends in

testicular cancer that can be seen as the ‘canary in the

coal mine’ for other spermatogenic disorders, are clearly

increasing. In addition, infertility due to poor semen

quality is widespread and the need for MAR, and use of

ICSI for male infertility, has become a costly issue and

a booming health industry in many parts of the world.

Also in favour of a biological hypothesis is the fact

that reproductive toxicants are ubiquitously present in

our diet, drinking water and the air we breathe

. It is

well established that these chemicals have become part

of our tissues and fluids. But do they contribute to the

current epidemic of infertility? We know that they can

be a threat to wildlife. Unfortunately, too little has been

done to uncover their role in humans.

For many societies in Asia and Europe, the popu-

lation situation is now rather dire. Countries/regions

with a rate of 1.5 children per woman (such as Japan

and Germany) have already seen a 50% reduction in the

number of babies born, and will (excluding migration)

face a further 50% reduction over the next 60 years if

current trends in fertility rates persist

(Fig. 9)

. South

Korea, with a fertility rate in 2020 of 0.84 will (excluding

migration) experience a 75% reduction in the number of

babies born within the next two generations, if current

birth rates persist

223

We urge governments, health authorities, including

WHO, and universities to seriously address the pros-

pects for human reproduction. If further analysis should

show that the reproductive trends can be explained by

socioeconomic and psychological factors alone, we

might not need to worry so much, as economic and

social factors often change. However, the trends seem to

have developed slowly over more than a century during

economic upturns and downturns. If the fertility prob-

lems are, at least partly, due to anthropogenic activities

that are causing increased environmental exposure to

harmful chemicals, in addition to effects on climate,

decisive regulatory actions underpinned by unconven-

tional, interdisciplinary research collaborations will be

needed to reverse the trends

(box 1)

Published online xx xx xxxx

Zegers-Hochschild, F. et al. The international glossary

on infertility and fertility care, 2017.

Hum. Reprod.

32,

1786–1801 (2017).

Priskorn, L., Dahl, C. L., Pihl, A. S., Skakkebaek, N. E.

& Juul, A. High maternal age at first and subsequent

child births in Denmark in the mid-1800s–Letter to

the editor.

Eur. J. Obstet. Gynecol. Reprod. Biol.

241,

137–138 (2019).

Lutz, W., O’Neill, B. C. & Scherbov, S. Demographics.

Europe’s population at a turning point.

Science

299,

1991–1992 (2003).

GBD 2017 Population and Fertility Collaborators.

Population and fertility by age and sex for 195

countries and territories, 1950-2017: a systematic

analysis for the Global Burden of Disease Study 2017.

Lancet

392,

1995–2051 (2018).

Nature reviews

Endocrinology

0123456789();:

SUU, Alm.del - 2021-22 - Bilag 154: Henvendelse af 14/1-22 fra Niels E. Skakkebæk m.fl. om forebyggelse af ufrugtbarhed

Reviews

Fellman, J. & Eriksson, A. W. Temporal differences

in the regional twinning rates in Sweden after 1750.

Twin Res.

183–191 (2003).

Blomberg, J. M., Priskorn, L., Jensen, T. K., Juul, A.

& Skakkebaek, N. E. Temporal trends in fertility rates:

a vationwide registry based study from 1901 to 2014.

PloS ONE

10,

e0143722 (2015).

Lackie, E. & Fairchild, A. The birth control pill,

thromboembolic disease, science and the media:

a historical review of the relationship.

Contraception

94,

295–302 (2016).

Sandström, G., Marklund, E. Fertility differentials in

Sweden during the first half of the twentieth century:

the changing effect of female labor force participation

and occupational field. Presented at the Annual

Meeting of the Population Association of America,

Chicago, 27–29 April 2017.

Skakkebaek, N. E. et al. Populations, decreasing

fertility, and reproductive health.

Lancet

393,

1500–1501 (2019).

Oeppen, J. & Vaupel, J. W. Demography. Broken limits

to life expectancy.

Science

296,

1029–1031 (2002).

Statistics Bureau of Japan. Japan’s Population

Estimates Released

https://www.stat.go.jp/english/

info/news/1910.html

(2010).

Tillotson, J. E. America’s obesity: conflicting public

policies, industrial economic development, and

unintended human consequences.

Annu. Rev. Nutr.

24,

617–643 (2004).

Haagen-Smit, A. J. A lesson from the smog capital

of the world.

Proc. Natl Acad. Sci. USA

67,

887–897

(1970).

Wang, F., Zheng, P., Dai, J., Wang, H. & Wang, R.

Fault tree analysis of the causes of urban smog

events associated with vehicle exhaust emissions:

a case study in Jinan, China.

Sci. Total. Environ.

668,

245–253 (2019).

World Health Organization. State of the Science of

Endocrine Disputing Chemicals – 2012.

https://www.

unep.org/resources/publication/state-science-

endocrine-disputing-chemicals-ipcp-2012

(2013).

Crinnion, W. J. The CDC fourth national report on

human exposure to environmental chemicals: what

it tells us about our toxic burden and how it assist

environmental medicine physicians.

Altern. Med. Rev.

15,

101–109 (2010).

Bergman, A. et al. The impact of endocrine disruption:

a consensus statement on the state of the science.

Environ. Health Perspect.

121,

A104–A106 (2013).

Andersson, A. M. et al. Adverse trends in male

reproductive health: we may have reached a crucial

‘tipping point’.

Int. J. Androl.

31,

74–80 (2008).

Christin-Maitre, S. History of oral contraceptive

drugs and their use worldwide.

Best. Pract. Res. Clin.

Endocrinol. Metab.

27,

3–12 (2013).

Mears, E. Clinical trials of oral contraceptives.

Br. Med. J.

1179–1183 (1961).

Finer, L. B. & Zolna, M. R. Declines in unintended

pregnancy in the United States, 2008-2011.

N. Engl.

J. Med.

374,

843–852 (2016).

Mumford, S. L., Sapra, K. J., King, R. B., Louis, J. F.

& Buck Louis, G. M. Pregnancy intentions–a complex

construct and call for new measures.

Fertil. Steril.

106,

1453–1462 (2016).

Sedgh, G. et al. Abortion incidence between 1990

and 2014: global, regional, and subregional

levels and trends.

Lancet

388,

258–267 (2016).

Jatlaoui, T. C. et al. Abortion surveillance – United

States, 2016.

MMWR Surveill. Summ.

68,

1–41

(2019).

Jensen, T. K. et al. Declining trends in conception

rates in recent birth cohorts of native Danish women:

a possible role of deteriorating male reproductive

health.

Int. J. Androl.

31,

81–92 (2008).

Lassen, T. H. et al. Trends in rates of natural

conceptions among Danish women born during

1960-1984.

Hum. Reprod.

27,

2815–2822

(2012).

Hognert, H. et al. High birth rates despite easy

access to contraception and abortion: a cross-sectional

study.

Acta Obstet. Gynecol. Scand.

96,

1414–1422

(2017).

Lidegaard, Ø. et al. Pregnancy loss: A 40-year

nationwide assessment.

Acta Obstet. Gynecol. Scand.

99,

1492–1496 (2020).

Rossen, L. M., Ahrens, K. A. & Branum, A. M.

Trends in risk of pregnancy loss among US women,

1990-2011.

Paediatr. Perinat. Epidemiol.

32,

19–29

(2018).

Tong, S. & Short, R. V. Dizygotic twinning as a

measure of human fertility.

Hum. Reprod.

13,

95–98

(1998).

33.

Asklund, C. et al. Twin pregnancy possibly associated

with high semen quality.

Hum. Reprod.

22,

751–755

(2007).

34.

Pison, G., Monden, C. & Smits, J. Twinning rates

in developed countries: trends and explanations.

Popul. Dev. Rev.

41,

629–649 (2015).

35.

Präg, P. & Mills, M. C. in

Childlessness in

Europe: Contexts, Causes, and Consequences

(eds Kreyenfeld, M. & Konietzka, D.) 289–309

(Springer, 2017).

36.

Bracken, M. B. Oral contraception and twinning:

an epidemiologic study.

Am. J. Obstet. Gynecol.

133,

432–434 (1979).

37.

Rachootin, P. & Olsen, J. Secular changes in the

twinning rate in Denmark 1931 to 1977.

Scand. J.

Soc. Med.

89–94 (1980).

38.

Olsen, J. & Rachootin, P. The end of the decline in

twinning rates?

Scand. J. Soc. Med.

11,

119 (1983).

39.

Andersen, A. N. & Erb, K. Register data on assisted

reproductive technology (ART) in Europe including a

detailed description of ART in Denmark.

Int. J. Androl.

29,

12–16 (2006).

40.

De Geyter, C. et al. ART in Europe, 2015: results

generated from European registries by ESHRE.

Hum. Reprod. Open

2020,

hoz038 (2020).

41.

Kamphuis, E. I., Bhattacharya, S., van der Veen, F.,

Mol, B. W. & Templeton, A. Are we overusing IVF?

BMJ

348,

g252 (2014).

42.

Sundhedsdatastyrelsen. Assisteret Reproduktion

2019.

https://docplayer.dk/204335156-

Assisteret-reproduktion-2019.html

(2019).

43.

Garcia, D., Brazal, S., Rodriguez, A., Prat, A. &

Vassena, R. Knowledge of age-related fertility decline

in women: a systematic review.

Eur. J. Obstet.

Gynecol. Reprod. Biol.

230,

109–118 (2018).

44.

Habbema, J. D., Eijkemans, M. J., Leridon, H. &

te Velde, E. R. Realizing a desired family size: when

should couples start?

Hum. Reprod.

30,

2215–2221

(2015).

45.

Hassan, M. A. & Killick, S. R. Effect of male age

on fertility: evidence for the decline in male fertility

with increasing age.

Fertil. Steril.

(Suppl 3),

1520–1527 (2003).

46.

Tsao, C. W. et al. Exploration of the association

between obesity and semen quality in a 7630 male

population.

PLoS ONE

10,

e0119458 (2015).

47.

Nieschlag, E., Lammers, U., Freischem, C. W.,

Langer, K. & Wickings, E. J. Reproductive functions

in young fathers and grandfathers.

J. Clin. Endocrinol.

Metab.

55,

676–681 (1982).

48.

Ge, Z. J., Schatten, H., Zhang, C. L. & Sun, Q. Y.

Oocyte ageing and epigenetics.

Reproduction

149,

R103–R114 (2015).

49.

Gruhn, J. R. et al. Chromosome errors in human

eggs shape natural fertility over reproductive life span.

Science

365,

1466–1469 (2019).

50.

Handyside, A. H. Molecular origin of female

meiotic aneuploidies.

Biochim. Biophys. Acta

1822,

1913–1920 (2012).

51.

Newman, J. E., Fitzgerlad, O., Paul, R. C. &

Chambers, G. M. Assisted reproductive technology

in Australia and New Zealand 2017.

https://npesu.

unsw.edu.au/sites/default/files/npesu/data_collection/

Assisted%20Reproductive%20Technology%20in%20

Australia%20and%20New%20Zealand%202017.pdf

(2019).

52.

Neels, K., Murphy, M., Ni Bhrolchain, M. &

Beaujouan, E. Rising educational participation and

the trend to later childbearing.

Popul. Dev. Rev.

43,

667–693 (2017).

53.

Joham, A. E., Palomba, S. & Hart, R. Polycystic ovary

syndrome, obesity, and pregnancy.

Semin. Reprod.

Med.

34,

93–101 (2016).

54.

Koninckx, P. R. et al. The epidemiology of

endometriosis is poorly known as the pathophysiology

and diagnosis are unclear.

Best. Pract. Res. Clin.

Obstet. Gynaecol.

71,

14–26 (2021).

55.

Noriega, N. C., Ostby, J., Lambright, C., Wilson, V. S.

& Gray, L. E. Jr. Late gestational exposure to the

fungicide prochloraz delays the onset of parturition

and causes reproductive malformations in male

but not female rat offspring.

Biol. Reprod.

72,

1324–1335 (2005).

56.

Buck Louis, G. M. et al. Paternal exposures to

environmental chemicals and time-to-pregnancy:

overview of results from the LIFE study.

Andrology

639–647 (2016).

57.

Lum, K. J., Sundaram, R., Barr, D. B., Louis, T. A. &

Buck Louis, G. M. Perfluoroalkyl chemicals, menstrual

cycle length, and fecundity: findings from a

prospective pregnancy study.

Epidemiology

28,

90–98 (2017).

58.

Mínguez-Alarcón, L. & Gaskins, A. J. Female exposure

to endocrine disrupting chemicals and fecundity:

a review.

Curr. Opin. Obstet. Gynecol.

29,

202–211

(2017).

59.

Buck Louis, G. M., Kannan, K., Sapra, K. J.,

Maisog, J. & Sundaram, R. Urinary concentrations

of benzophenone-type ultraviolet radiation filters

and couples’ fecundity.

Am. J. Epidemiol.

180,

1168–1175 (2014).

60.

Smarr, M. M., Sundaram, R., Honda, M., Kannan, K.

& Louis, G. M. Urinary concentrations of parabens

and other antimicrobial chemicals and their

association with couples’ fecundity.

Environ. Health

Perspect.

125,

730–736 (2017).

61.

Abu-Halima, M. et al. Panel of five microRNAs

as potential biomarkers for the diagnosis and

assessment of male infertility.

Fertil. Steril.

102,

989–997 (2014).

62.

Steinmetz, R., Brown, N. G., Allen, D. L., Bigsby, R. M.

& Ben-Jonathan, N. The environmental estrogen

bisphenol A stimulates prolactin release in vitro and

in vivo.

Endocrinology

138,

1780–1786 (1997).

63.

Steinmetz, R. et al. The xenoestrogen bisphenol A

induces growth, differentiation, and c-fos gene

expression in the female reproductive tract.

Endocrinology

139,

2741–2747 (1998).

64.

Spearow, J. L., Doemeny, P., Sera, R., Leffler, R. &

Barkley, M. Genetic variation in susceptibility to

endocrine disruption by estrogen in mice.

Science

285,

1259–1261 (1999).

65.

Spearow, J. L. et al. Genetic variation in physiological

sensitivity to estrogen in mice.

APMIS

109,

356–364

(2001).

66.

Spearow, J. L. & Barkley, M. Reassessment of models

used to test xenobiotics for oestrogenic potency

is overdue.

Hum. Reprod.

16,

1027–1029 (2001).

67.

Amann, R. P. & Howards, S. S. Daily spermatozoal

production and epididymal spermatozoal reserves

of the human male.

J. Urol.

124,

211–215 (1980).

68.

Franca, L. R., Russell, L. D. & Cummins, J. M. Is

human spermatogenesis uniquely poor?

Ann. Rev.

Biomed. Sci.

19–40 (2002).

69.

Short, R. V. The testis: the witness of the mating

system, the site of mutation and the engine of desire.

Acta Paediatr. Suppl.

422,

3–7 (1997).

70.

Hess, R. A. & França, L. R. in

Molecular Mechanisms

in Spermatogenesis

(ed. Cheng, C.) 1–15 (Landes

Bioscience, 2007).

71.

França, L. R., Ogawa, T., Avarbock, M. R., Brinster, R. L.

& Russell, L. D. Germ cell genotype controls cell cycle

during spermatogenesis in the rat.

Biol. Reprod.

59,

1371–1377 (1998).

72.

França, L. R., Avelar, G. F. & Almeida, F. F.

Spermatogenesis and sperm transit through the

epididymis in mammals with emphasis on pigs.

Theriogenology

63,

300–318 (2005).

73.

Carlsen, E., Giwercman, A., Keiding, N. &

Skakkebæk, N. E. Evidence for decreasing quality

of semen during past 50 years.

BMJ

305,

609–613

(1992).

74.

Levine, H. et al. Temporal trends in sperm count:

a systematic review and meta-regression analysis.

Hum. Reprod. Update

23,

646–659 (2017).

75.

Huang, C. et al. Decline in semen quality among

30,636 young Chinese men from 2001 to 2015.

Fertil. Steril.

107,

83–88.e2 (2017).

76.

Yuan, H. F. et al. Decline in semen concentration

of healthy Chinese adults: evidence from 9357

participants from 2010 to 2015.

Asian J. Androl.

20,

379–384 (2018).

77.

Huang, X. et al. Association of exposure to ambient

fine particulate matter constituents with semen

quality among men attending a fertility center in

China.

Environ. Sci. Technol.

53,

5957–5965 (2019).

78.

Priskorn, L. et al. Average sperm count remains

unchanged despite reduction in maternal smoking:

results from a large cross-sectional study with

annual investigations over 21 years.

Hum. Reprod.

33,

998–1008 (2018).

79.

Jørgensen, N. et al. Human semen quality in

the new millennium: a prospective cross-sectional

population-based study of 4867 men.

BMJ Open

e000990 (2012).

80.

Bonde, J. P. E. et al. Relation between semen quality

and fertility: a population-based study of 430

first-pregnancy planners.

Lancet

352,

1172–1177

(1998).

81.

Guzick, D. S. et al. Sperm morphology, motility, and

concentration in fertile and infertile men.

N. Engl. J.

Med.

345,

1388–1393 (2001).

82.

Slama, R. et al. Time to pregnancy and semen

parameters: a cross-sectional study among fertile

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

www.nature.com/nrendo

0123456789();:

SUU, Alm.del - 2021-22 - Bilag 154: Henvendelse af 14/1-22 fra Niels E. Skakkebæk m.fl. om forebyggelse af ufrugtbarhed

Reviews

couples from four European cities.

Hum. Reprod.

17,

503–515 (2002).

World Health Organization. WHO Laboratory

Manual for the Examination and Processing of Human

Semen.

https://www.who.int/publications/i/item/

9789240030787

(2021).

Skakkebaek, N. E. Normal reference ranges for

semen quality and their relations to fecundity.

Asian J.

Androl.

12,

95–98 (2010).

Skakkebæk, N. E., Rajpert-De Meyts, E. & Main, K. M.

Testicular dysgenesis syndrome: an increasingly

common developmental disorder with environmental

aspects.

Hum. Reprod.

16,

972–978 (2001).

Clemmesen, J. A doubling of morbidity from testis

carcinoma in Copenhagen, 1943–1962.

APMIS

72,

348–349 (1968).

Znaor, A. et al. Testicular cancer incidence predictions

in Europe 2010-2035: a rising burden despite

population ageing.

Int. J. Cancer

147,

820–828

(2020).

Møller, H. Clues to the aetiology of testicular germ cell

tumours from descriptive epidemiology.

Eur. Urol.

23,

8–15 (1993).

Bergström, R. et al. Increase in testicular cancer

incidence in six European countries: a birth cohort

phenomenon.

J. Natl Cancer Inst.

88,

727–733

(1996).

Grumet, R. F. & MacMahon, B. Trends in mortality

from neoplasms of the testis.

Cancer

11,

790–797

(1958).

Case, R. A. Cohort analysis of cancer mortality in

England and Wales; 1911–1954 by site and sex.

Br. J. Prev. Soc. Med.

10,

172–199 (1956).

Sung, H. et al. Global cancer statistics 2020:

GLOBOCAN estimates of incidence and mortality

worldwide for 36 cancers in 185 countries.

CA Cancer

J. Clin.

71,

209–249 (2021).

Bray, F. et al. Cancer incidence in five continents,

Vol. XI (electronic version). Lyon: International Agency

for Research on Cancer.

https://ci5.iarc.fr/CI5-XI/

Default.aspx

(2017).

International Agency for Research on Cancer. CI5plus:

Cancer Incidence in Five Continents Time Trends.

http://ci5.iarc.fr/CI5plus/Default.aspx

(2018).

Znaor, A., Lortet-Tieulent, J., Jemal, A. & Bray, F.

International variations and trends in testicular cancer

incidence and mortality.

Eur. Urol.

65,

1095–1106

(2014).

Harbuz, R. et al. A recurrent deletion of DPY19L2

causes infertility in man by blocking sperm head

elongation and acrosome formation.

Am. J. Hum.

Genet.

88,

351–361 (2011).

Dam, A. H. et al. Homozygous mutation in SPATA16

is associated with male infertility in human

globozoospermia.

Am. J. Hum. Genet.

81,

813–820

(2007).

Tüttelmann, F., Ruckert, C. & Röpke, A. Disorders

of spermatogenesis: perspectives for novel genetic

diagnostics after 20 years of unchanged routine.

Med. Genet.

30,

12–20 (2018).

Krausz, C. & Riera-Escamilla, A. Genetics of male

infertility.

Nat. Rev. Urol.

15,

369–384 (2018).

Nagirnaja, L. et al. Variant PNLDC1, defective piRNA

processing, and azoospermia.

N. Engl. J. Med.

385,

707–719 (2021).

Kasak, L. & Laan, M. Monogenic causes of

non-obstructive azoospermia: challenges, established

knowledge, limitations and perspectives.

Hum. Genet.

140,

135–154 (2020).

Barban, N. et al. Genome-wide analysis identifies

12 loci influencing human reproductive behavior.

Nat. Genet.

48,

1462–1472 (2016).

Stolk, L. et al. Loci at chromosomes 13, 19 and 20

influence age at natural menopause.

Nat. Genet.

41,

645–647 (2009).

Day, F. R. et al. Genomic analyses identify hundreds of

variants associated with age at menarche and support

a role for puberty timing in cancer risk.

Nat. Genet.

49,

834–841 (2017).

Ruth, K. S. et al. Genome-wide association study of

anti-Müllerian hormone levels in pre-menopausal

women of late reproductive age and relationship

with genetic determinants of reproductive lifespan.

Hum. Mol. Genet.

28,

1392–1401 (2019).

Stolk, L. et al. Meta-analyses identify 13 loci

associated with age at menopause and highlight

DNA repair and immune pathways.

Nat. Genet.

44,

260–268 (2012).

Lutzmann, M. et al. MCM8- and MCM9-deficient mice

reveal gametogenesis defects and genome instability

due to impaired homologous recombination.

Mol. Cell

47,

523–534 (2012).

108.

Cavalli, G. & Heard, E. Advances in epigenetics link

genetics to the environment and disease.

Nature

571,

489–499 (2019).

109.

Almstrup, K. et al. Pubertal development in healthy

children is mirrored by DNA methylation patterns

in peripheral blood.

Sci. Rep.

28657 (2016).

110.

Chen, S. et al. Age at onset of different pubertal signs

in boys and girls and differential DNA methylation at

age 10 and 18 years: an epigenome-wide follow-up

study.

Hum. Reprod. Open

2020,

hoaa006 (2020).

111.

Kresovich, J. K. et al. Reproduction, DNA methylation

and biological age.

Hum. Reprod.

34,

1965–1973

(2019).

112.

Meehan, R. R., Thomson, J. P., Lentini, A.,

Nestor, C. E. & Pennings, S. DNA methylation as

a genomic marker of exposure to chemical and

environmental agents.

Curr. Opin. Chem. Biol.

45,

48–56 (2018).

113.

Almstrup, K., Frederiksen, H., Andersson, A. M. &

Juul, A. Levels of endocrine-disrupting chemicals

are associated with changes in the peri-pubertal

epigenome.

Endocr. Connect.

845–857 (2020).

114.

Tobi, E. W. et al. DNA methylation signatures link

prenatal famine exposure to growth and metabolism.

Nat. Commun.

5592 (2014).

115.

Richmond, R. C. et al. Prenatal exposure to maternal

smoking and offspring DNA methylation across the

lifecourse: findings from the Avon Longitudinal Study

of Parents and Children (ALSPAC).

Hum. Mol. Genet.

24,

2201–2217 (2015).

116.

Leitão, E. et al. The sperm epigenome does not

display recurrent epimutations in patients with

severely impaired spermatogenesis.

Clin. Epigenet.

12,

61 (2020).

117.

Soubry, A. et al. Human exposure to flame-retardants

is associated with aberrant DNA methylation at

imprinted genes in sperm.

Environ. Epigenet.

dvx003 (2017).

118.

Wu, H. et al. Preconception urinary phthalate

concentrations and sperm DNA methylation

profiles among men undergoing IVF treatment:

a cross-sectional study.

Hum. Reprod.

32,

2159–2169

(2017).

119.

Greeson, K. W. et al. Detrimental effects of flame

retardant, PBB153, exposure on sperm and future

generations.

Sci. Rep.

10,

8567 (2020).

120.

Beck, D., Sadler-Riggleman, I. & Skinner, M. K.

Generational comparisons (F1 versus F3) of vinclozolin

induced epigenetic transgenerational inheritance

of sperm differential DNA methylation regions

(epimutations) using MeDIP-Seq.

Environ. Epigenet

dvx016 (2017).

121.

Nätt, D. & Öst, A. Male reproductive health and

intergenerational metabolic responses from a small

RNA perspective.

J. Intern. Med.

288,

305–320

(2020).

122.

Trigg, N. A., Eamens, A. L. & Nixon, B. The contribution

of epididymosomes to the sperm small RNA profile.

Reproduction

157,

R209–R223 (2019).

123.

Sharma, U. et al. Biogenesis and function of tRNA

fragments during sperm maturation and fertilization

in mammals.

Science

351,

391–396 (2016).

124.

Grandjean, V. et al. RNA-mediated paternal heredity

of diet-induced obesity and metabolic disorders.

Sci. Rep.

18193 (2015).

125.

Xu, H. et al. MicroRNA expression profile analysis in

sperm reveals hsa-mir-191 as an auspicious omen of

in vitro fertilization.

BMC Genomics

21,

165 (2020).

126.

Kong, A. et al. Rate of de novo mutations and the

importance of father’s age to disease risk.

Nature

488,

471–475 (2012).

127.

Nybo Andersen, A. M. & Urhoj, S. K. Is advanced

paternal age a health risk for the offspring?

Fertil.

Steril.

107,

312–318 (2017).

128.

Whorton, D., Milby, T. H., Krauss, R. M. & Stubbs, H. A.

Testicular function in DBCP exposed pesticide workers.

J. Occup. Med.

21,

161–166 (1979).

129.

Goldsmith, J. R., Potashnik, G. & Israeli, R.

Reproductive outcomes in families of DBCP-exposed

men.

Arch. Environ. Health

39,

85–89 (1984).

130.

Potashnik, G., Goldsmith, J. & Insler, V.

Dibromochloropropane-induced reduction of the

sex-ratio in man.

Andrologia

16,

213–218 (1984).

131.

Skakkebaek, N. E. et al. Male reproductive disorders

and fertility trends: influences of environment and

genetic susceptibility.

Physiol. Rev.

96,

55–97

(2016).

132.

Messiaen, S. et al. Rad54 is required for the normal

development of male and female germ cells and

contributes to the maintainance of their genome

integrity after genotoxic stress.

Cell Death Dis.

e774 (2013).

133.

Mandl, A. M., Beaumont, H. M. & Hughes, G. C.

Effects of Ionizing Radiation on the Reproductive

System

(eds Carlson, W. D. & Gassner, F. X.) 165

(Pergamon Press, 1964).

134.

Mandl, A. M. The radiosensitivity of germ cells.

Biol. Rev.

39,

288–371 (1964).

135.

Gray, L. E. Jr. et al. Perinatal exposure to the

phthalates DEHP, BBP, and DINP, but not DEP, DMP,

or DOTP, alters sexual differentiation of the male rat.

Toxicol. Sci.

58,

350–365 (2000).

136.

Fisher, J. S., Macpherson, S., Marchetti, N. &

Sharpe, R. M. Human “testicular dysgenesis

syndrome”: a possible model using in-utero exposure

of the rat to dibutyl phthalate.

Hum. Reprod.

18,

1383–1394 (2003).

137.

Gray, L. E. Jr., Ostby, J. S. & Kelce, W. R. Developmental

effects of an environmental antiandrogen: the

fungicide vinclozolin alters sex differentiation of the

male rat.

Toxixol. Appl. Pharmacol.

129,

46–52

(1994).

138.

Hass, U. et al. Combined exposure to anti-androgens

exacerbates disruption of sexual differentiation

in the rat.

Environ. Health Perspect.

115

(Suppl 1),

122–128 (2007).

139.

Welsh, M. et al. Identification in rats of a programming

window for reproductive tract masculinization,

disruption of which leads to hypospadias and

cryptorchidism.

J. Clin. Invest.

118,

1479–1490

(2008).

140.

Van den Driesche, S. et al. Experimentally induced

testicular dysgenesis syndrome originates in the

masculinization programming window.

JCI Insight

e91204 (2017).

141.

Kortenkamp, A. Which chemicals should be grouped

together for mixture risk assessments of male

reproductive disorders?

Mol. Cell Endocrinol.

499,

110581 (2020).

142.

Howdeshell, K. L., Hotchkiss, A. K. & Gray, L. E. Jr

Cumulative effects of antiandrogenic chemical

mixtures and their relevance to human health

risk assessment.

Int. J. Hyg. Environ. Health

220,

179–188 (2017).

143.

Gray, L. E. Jr & Ostby, J. S. In utero

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters

reproductive morphology and function in female rat

offspring.

Toxixol. Appl. Pharmacol.

133,

285–294

(1995).

144.

Lovekamp-Swan, T. & Davis, B. J. Mechanisms of

phthalate ester toxicity in the female reproductive

system.

Environ. Health Perspect.

111,

139–145

(2003).

145.

Guerra, M. T., Scarano, W. R., de Toledo, F. C.,

Franci, J. A. & Kempinas Wde, G. Reproductive

development and function of female rats exposed

to di-eta-butyl-phthalate (DBP) in utero and during

lactation.

Reprod. Toxicol.

29,

99–105 (2010).

146.

Johansson, H. K. L. et al. Putative adverse outcome

pathways for female reproductive disorders to improve

testing and regulation of chemicals.

Arch. Toxicol.

94,

3359–3379 (2020).

147.

Mocarelli, P. et al. Perinatal exposure to low doses of

dioxin can permanently impair human semen quality.

Environ. Health Perspect.

119,

713–718 (2011).

148.

Hardell, L., van Bavel, B., Lindstrom, G., Eriksson, M.

& Carlberg, M. In utero exposure to persistent organic

pollutants in relation to testicular cancer risk.

Int. J.

Androl.

29,

228–234 (2006).

149.

Krysiak-Baltyn, K. et al. Association between chemical

pattern in breast milk and congenital cryptorchidism:

modelling of complex human exposures.

Int. J. Androl.

35,

294–302 (2012).

150.

Main, K. M. et al. Flame retardants in placenta

and breast milk and cryptorchidism in newborn

boys.

Environ. Health Perspect.

115,

1519–1526

(2007).

151.

Hemminki, K. & Li, X. Cancer risks in Nordic

immigrants and their offspring in Sweden.

Eur. J.

Cancer

38,

2428–2434 (2002).

152.

Schmiedel, S., Schuz, J., Skakkebæk, N. E. &

Johansen, C. Testicular germ cell cancer incidence in

an immigration perspective, Denmark, 1978 to 2003.

J. Urol.

183,

1378–1382 (2010).

153.

Myrup, C. et al. Testicular cancer risk in first- and

second-generation immigrants to Denmark.

J. Natl

Cancer Inst.

100,

41–47 (2008).

154.

Nielsen, H., Nielsen, M. & Skakkebæk, N. E.

The fine structure of a possible carcinoma-in-situ

in the seminiferous tubules in the testis of four

infertile men.

APMIS

82,

235–248 (1974).

155.

Almstrup, K. et al. Genomic and gene expression

signature of the pre-invasive testicular carcinoma

in situ.

Cell Tissue Res.

322,

159–165 (2005).

83.

84.

85.

86.

87.

88.

89.

90.

91.

92.

93.

94.

95.

96.

97.

98.

99.

100.

101.

102.

103.

104.

105.

106.

107.

Nature reviews

Endocrinology

0123456789();:

SUU, Alm.del - 2021-22 - Bilag 154: Henvendelse af 14/1-22 fra Niels E. Skakkebæk m.fl. om forebyggelse af ufrugtbarhed

Reviews

156.

Almstrup, K. et al. Embryonic stem cell-like features

of testicular carcinoma in situ revealed by

genome-wide gene expression profiling.

Cancer Res.

64,

4736–4743 (2004).

157.

Moch, H., Cubilla, A. L., Humphrey, P. A., Reuter, V. E.

& Ulbright, T. M. The 2016 WHO classification of

tumours of the urinary system and male genital

organs–part a: renal, penile, and testicular tumours.

Eur. Urol.

70,

93–105 (2016).

158.

Sharpe, R. M. & Skakkebæk, N. E. Testicular

dysgenesis syndrome: mechanistic insights and

potential new downstream effects.

Fertil. Steril.

89,

e33–e38 (2008).

159.

Lottrup, G. et al. Identification of a novel androgen

receptor mutation in a family with multiple components

compatible with the testicular dysgenesis syndrome.

J. Clin. Endocrinol. Metab.

98,

2223–2229 (2013).

160.

Depue, R. H., Pike, M. C. & Henderson, B. E.

Cryptorchidism and testicular cancer.

J. Natl Cancer

Inst.

77,

830–832 (1986).

161.

Moller, H. & Skakkebæk, N. E. Risks of testicular

cancer and cryptorchidism in relation to socio-economic

status and related factors: case-control studies in

Denmark.

Int. J. Cancer

66,

287–293 (1996).

162.

Krabbe, S. et al. High incidence of undetected

neoplasia in maldescended testes.

Lancet

313,

999–1000 (1979).

163.

Serrano, T., Chevrier, C., Multigner, L., Cordier, S. &

Jegou, B. International geographic correlation study

of the prevalence of disorders of male reproductive

health.

Hum. Reprod.

28,

1974–1986 (2013).

164.

Jørgensen, N. et al. East-West gradient in semen

quality in the Nordic-Baltic area: a study of men

from the general population in Denmark, Norway,

Estonia and Finland.

Hum. Reprod.

17,

2199–2208

(2002).

165.

Berthelsen, J. G. & Skakkebæk, N. E. Gonadal

function in men with testis cancer.

Fertil. Steril.

39,

68–75 (1983).

166.

Berthelsen, J. G.

Andrological Aspects of Testicular

Cancer

9–44 (Scriptor, 1984).

167.

Petersen, P. M. et al. Impaired testicular function in

patients with carcinoma in situ of the testis.

J. Clin.

Oncol.

17,

173–179 (1999).

168.

Jacobsen, R. et al. Risk of testicular cancer in men

with abnormal semen characteristics: cohort study.

Br. Med. J.

321,

789–792 (2000).

169.

Andersson, A. M. et al. Secular decline in male

testosterone and sex hormone binding globulin serum

levels in Danish population surveys.

J. Clin. Endocrinol.

Metab.

92,

4696–4705 (2007).

170.

Travison, T. G., Araujo, A. B., O’Donnell, A. B.,

Kupelian, V. & McKinlay, J. B. A population-level

decline in serum testosterone levels in American

men.

J. Clin. Endocrinol. Metab.

92,

196–202

(2007).

171.

Sharma, R., Harlev, A., Agarwal, A. & Esteves, S. C.

Cigarette smoking and semen quality: a new

meta-analysis examining the effect of the 2010 World

Health Organization Laboratory Methods for

the Examination of Human Semen.

Eur. Urol.

70,

635–645 (2016).

172.

Jensen, T. K. et al. Association of in utero exposure

to maternal smoking with reduced semen quality and

testis size in adulthood: a cross-sectional study of

1,770 young men from the general population in five

European countries.

Am. J. Epidemiol.

159,

49–58

(2004).

173.

Ramlau-Hansen, C. H. et al. Is prenatal exposure

to tobacco smoking a cause of poor semen quality?

A follow-up study.

Am. J. Epidemiol.

165,

1372–1379

(2007).

174.

Jensen, T. K. et al. Adult and prenatal exposures

to tobacco smoke as risk indicators of fertility

among 430 Danish couples.

Am. J. Epidemiol.

148,

992–997 (1998).

175.

Radin, R. G. et al. Active and passive smoking and

fecundability in Danish pregnancy planners.

Fertil.

Steril.

102,

183–191.e2 (2014).

176.

Sapra, K. J., Barr, D. B., Maisog, J. M., Sundaram, R.

& Buck Louis, G. M. Time-to-pregnancy associated

with couples’ use of tobacco products.

Nicotine Tob. Res.

18,

2154–2161 (2016).

177.

Wesselink, A. K. et al. Prospective study of cigarette

smoking and fecundability.

Hum. Reprod.

34,

558–567 (2019).

178.

Holmboe, S. A. et al. Use of e-cigarettes associated

with lower sperm counts in a cross-sectional study

of young men from the general population.

Hum.

Reprod.

35,

1693–1701 (2020).

179.

Gundersen, T. D. et al. Association between use of

marijuana and male reproductive hormones and

semen quality: a study among 1,215 healthy

young men.

Am. J. Epidemiol.

182,

473–481

(2015).

180.

Nassan, F. L. et al. Marijuana smoking and markers

of testicular function among men from a fertility

centre.

Hum. Reprod.

34,

715–723 (2019).

181.

Jensen, T. K. et al. Does moderate alcohol consumption

affect fertility? Follow up study among couples planning

first pregnancy.

BMJ

317,

505–510 (1998).

182.

Ricci, E. et al. Semen quality and alcohol intake:

a systematic review and meta-analysis.

Reprod.

Biomed. Online

34,

38–47 (2017).

183.

Salas-Huetos, A., James, E. R., Aston, K. I.,

Jenkins, T. G. & Carrell, D. T. Diet and sperm quality:

nutrients, foods and dietary patterns.

Reprod. Biol.

19,

219–224 (2019).

184.

Nassan, F. L. et al. Association of dietary patterns with

testicular function in young Danish men.

JAMA Netw.

Open

e1921610 (2020).

185.

Grieger, J. A. et al. Pre-pregnancy fast food and

fruit intake is associated with time to pregnancy.

Hum. Reprod.

33,

1063–1070 (2018).

186.

Lee, S., Min, J. Y., Kim, H. J. & Min, K. B. Association

between the frequency of eating non-home-

prepared meals and women infertility in the

United States.

J. Prev. Med. Public Health

53,

73–81

(2020).

187.

Hatch, E. E. et al. Intake of sugar-sweetened

beverages and fecundability in a North American

preconception cohort.

Epidemiology

29,

369–378

(2018).

188.

Supramaniam, P. R., Mittal, M., McVeigh, E. &

Lim, L. N. The correlation between raised body mass

index and assisted reproductive treatment outcomes:

a systematic review and meta-analysis of the evidence.

Reprod. Health

15,

34 (2018).

189.

Mushtaq, R. et al. Effect of male body mass index on

assisted reproduction treatment outcome: an updated

systematic review and meta-analysis.

Reprod. Biomed.

Online

36,

459–471 (2018).

190.

Sermondade, N. et al. BMI in relation to sperm count:

an updated systematic review and collaborative

meta-analysis.

Hum. Reprod. Update

19,

221–231

(2013).

191.

Ma, J. et al. Association between BMI and semen

quality: an observational study of 3966 sperm

donors.

Hum. Reprod.

34,

155–162 (2019).

192.

Vaamonde, D., Da Silva-Grigoletto, M. E.,

Garcia-Manso, J. M., Barrera, N. & Vaamonde-Lemos, R.

Physically active men show better semen parameters

and hormone values than sedentary men.

Eur. J. Appl.

Physiol.

112,

3267–3273 (2012).

193.

Gaskins, A. J. et al. Physical activity and television

watching in relation to semen quality in young men.

Br. J. Sports Med.

49,

265–270 (2015).

194.

Lalinde-Acevedo, P. C. et al. Physically active men

show better semen parameters than their sedentary

counterparts.

Int. J. Fertil. Steril.

11,

156–165

(2017).

195.

Sun, B. et al. Physical activity and sedentary time

in relation to semen quality in healthy men screened

as potential sperm donors.

Hum. Reprod.

34,

2330–2339 (2019).

196.

Hajizadeh Maleki, B. & Tartibian, B. Moderate aerobic

exercise training for improving reproductive function

in infertile patients: a randomized controlled trial.

Cytokine

92,

55–67 (2017).

197.

Ritchie, H. & Roser, M. Fossil fuels.

Our World

in Data

https://ourworldindata.org/fossil-fuels

(2020).

198.

Thompson, R. C., Moore, C. J., vom Saal, F. S. &

Swan, S. H. Plastics, the environment and human

health: current consensus and future trends.

Philos.

Trans. R. Soc. Lond. B Biol. Sci.

364,

2153–2166

(2009).

199.

Francis, M. About 7% of fossil fuels are consumed for

non-combustion use in the United States.

U.S. Energy

Information System

https://www.eia.gov/

todayinenergy/detail.php?id=35672

(2018).

200.

Festel, G., Evans, D. & Jackson, B. Trade sustainability

impact assessment for the negotiations of a

partnership and cooperation agreement between

the EU and China.

https://trade.ec.europa.eu/

doclib/docs/2008/september/tradoc_140583.pdf

(2008).

201.

Woodruff, T. J., Zota, A. R. & Schwartz, J. M.

Environmental chemicals in pregnant women in the

US: NHANES 2003-2004.

Environ. Health Perspect.

119,

878–885 (2011).

202.

Rogan, W. J. et al. Polychlorinated biphenyls (PCBs)

and dichlorodiphenyl dichloroethene (DDE) in

human milk: effects of maternal factors and

previous lactation.

Am. J. Public Health

76,

172–177

(1986).

203.

Fang, J., Nyberg, E., Bignert, A. & Bergman, A.

Temporal trends of polychlorinated dibenzo-p-dioxins

and dibenzofurans and dioxin-like polychlorinated

biphenyls in mothers’ milk from Sweden, 1972-2011.

Environ. Int.

60,

224–231 (2013).

204.

Frederiksen, H. et al. UV filters in matched seminal

fluid-, urine-, and serum samples from young

men.

J. Expo. Sci. Environ. Epidemiol.

31,

345–355

(2020).

205.

Apel, P. et al. Time course of phthalate cumulative

risks to male developmental health over a 27-year

period: biomonitoring samples of the German

Environmental Specimen Bank.

Environ. Int.

137,

105467 (2020).

206.

Lewtas, J. Air pollution combustion emissions:

characterization of causative agents and mechanisms

associated with cancer, reproductive, and cardiovascular

effects.

Mutat. Res.

636,

95–133 (2007).

207.

Vohra, K. et al. Global mortality from outdoor fine

particle pollution generated by fossil fuel combustion:

results from GEOS-Chem.

Environ. Res.

195,

110754

(2021).

208.

Bamberger, M. et al. Surface water and groundwater

analysis using aryl hydrocarbon and endocrine

receptor biological assays and liquid chromatography-

high resolution mass spectrometry in Susquehanna

County, PA.

Environ. Sci. Process. Impacts

21,

988–998 (2019).

209.

Harville, E. W., Shankar, A., Zilversmit, L. &

Buekens, P. The Gulf oil spill, miscarriage, and

infertility: the GROWH study.

Int. Arch. Occup.

Environ. Health

91,

47–56 (2018).

210.

Mocarelli, P., Brambilla, P., Gerthoux, P. M.,

Patterson, D. G. Jr & Needham, L. L. Change in sex

ratio with exposure to dioxin.

Lancet

348,

409–409

(1996).

211.

Silva, M. J. et al. Urinary levels of seven phthalate

metabolites in the U.S. population from the National

Health and Nutrition Examination Survey (NHANES)

1999-2000.

Environ. Health Perspect.

112,

331–338

(2004).

212.

Colborn, T. & Clement, C.

Chemically-Induced

Alterations in Sexual and Functional Development:

the Wildlife/Human Connection

(Princeton Scientific,

1992).

213.

Baskin, L. S., Himes, K. & Colborn, T. Hypospadias

and endocrine disruption: is there a connection?

Environ. Health Perspect.

109,

1175–1183

(2001).

214.

Hauser, R. et al. Male reproductive disorders,

diseases, and costs of exposure to endocrine-

disrupting chemicals in the European union.

J. Clin.

Endocrinol. Metab.

100,

1267–1277 (2015).

215.

Rajpert-De Meyts, E., McGlynn, K. A., Okamoto, K.,

Jewett, M. A. & Bokemeyer, C. Testicular germ cell

tumours.

Lancet

387,

1762–1774 (2016).

216.

Sallmén, M., Weinberg, C. R., Baird, D. D.,

Lindbohm, M. L. & Wilcox, A. J. Has human fertility

declined over time? Why we may never know.

Epidemiology

16,

494–499 (2005).

217.

Joffe, M. et al. Studying time to pregnancy by use

of a retrospective design.

Am. J. Epidemiol.

162,

115–124 (2005).

218.

Ahrenfeldt, L. J. et al. Heritability of subfertility

among Danish twins.

Fertil. Steril.

114,

618–627

(2020).

219.

Buck, G. M. et al. Prospective pregnancy study designs

for assessing reproductive and developmental toxicants.

Environ. Health Perspect.

112,

79–86 (2004).

220.

Scheike, T. H. & Keiding, N. Design and analysis

of time-to-pregnancy.

Stat. Methods Med. Res.

15,

127–140 (2006).

221.

Slama, R. et al. Feasibility of the current-duration

approach to studying human fecundity.

Epidemiology

17,

440–449 (2006).

222.

Hatch, E. E. et al. Evaluation of selection bias in

an internet-based study of pregnancy planners.

Epidemiology

27,

98–104 (2016).

223.

KOSIS. Vital statistics of Korea.

https://kosis.kr/

statHtml/statHtml.do?orgId=101&tblId=DT_

1B8000F&language=en

(2021).

224.

Leal, M. C. & França, L. R. The seminiferous

epithelium cycle length in the black tufted-ear

marmoset (Callithrix penicillata) is similar to humans.

Biol. Reprod.

74,

616–624 (2006).

225.

de Oliveira, C. F. A., Lara, N., Cardoso, B. R. L.,

de França, L. R. & de Avelar, G. F. Comparative

testis structure and function in three representative

mice strains.

Cell Tissue Res.

382,

391–404

(2020).

www.nature.com/nrendo

0123456789();:

SUU, Alm.del - 2021-22 - Bilag 154: Henvendelse af 14/1-22 fra Niels E. Skakkebæk m.fl. om forebyggelse af ufrugtbarhed

Reviews

226.

Garner, D. L. & Hafez, E. S. E. Spermatozoa and

seminal plasma. in

Reproduction in Farm Animals

(ed. Hafez, E. S. E.) 165–187 (Lea and Febiger,

1993).

227.

Valle Rdel, R. et al. Semen characteristics of captive

common marmoset (Callithrix jacchus): a comparison

of a German with a Brazilian colony.

J. Med. Primatol.

43,

225–230 (2014).

228.

Bezerra, M. J. B. et al. Major seminal plasma

proteome of rabbits and associations with

sperm quality.

Theriogenology

128,

156–166

(2019).

229.

Okamura, A. et al. Broken sperm, cytoplasmic

droplets and reduced sperm motility are principal

markers of decreased sperm quality due to

organophosphorus pesticides in rats.

J. Occup. Health

51,

478–487 (2009).

230.

Prins, G. in

Encyclopedia of Reproduction

Vol. 4 (eds

Knobil, E. & Neill, J. D.) 360–367 (Academic, 1998).

231.

Bhattacharjee, R. et al. Targeted disruption of

glycogen synthase kinase 3A (GSK3A) in mice

affects sperm motility resulting in male infertility.

Biol. Reprod.

92,

65 (2015).

232.

Harris, T., Marquez, B., Suarez, S. & Schimenti, J.

Sperm motility defects and infertility in male mice with

a mutation in Nsun7, a member of the Sun domain-

containing family of putative RNA methyltransferases.

Biol. Reprod.

77,

376–382 (2007).

Acknowledgements

We thank M. Laversanne from the International Agency for

Research on Cancer for assistance with the first draft of Fig. 4.

Where authors are identified as personnel of the International

Agency for Research on Cancer/World Health Organization,

the authors alone are responsible for the views expressed in

this article and they do not necessarily represent the decisions,

policy or views of the International Agency for Research on

Cancer/World Health Organization. The authors acknowledge

financial support from the Innovation Fund Denmark, Danish

Ministry of Environment (CEHOS), Danish Ministry of Health,

The ReproUnion consortium/EU Interreg ÖKS, Brazilian insti-

tutions (CNPq and CAPES) and Minas Gerais State Foundation

(FAPEMIG). Finally, the authors thank A. Wahlberg,

Department of Anthropology, University of Copenhagen, and

K. Kjær, GLOBE Institute, University of Copenhagen, for most

valuable discussions prior to writing the paper.

Competing interests

R.J.H. is the Medical Director of Fertility Specialists of

Western Australia and has equity interests in Western IVF.

The other authors declare no competing interests.

Peer review information

Nature Reviews Endocrinology

thanks A.-S. Parent and the

other, anonymous, reviewer(s) for their contribution to

the peer review of this work.

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Supplementary information

The online version contains supplementary material available

https://doi.org/10.1038/s41574-021-00598-8.

Author contributions

N.E.S., R.L.-J., A.-M.A., S.A.H., E.V.B., K.A., L.R.F., A.Z., R.J.H.

and A.J. researched data for the article, contributed substan-

tially to discussion of the content, wrote the article and reviewed

and/or edited the manuscript before submission. H.L., N.J.,

K.M.M., Ø.L. and A.K. contributed substantially to discussion

of the content, wrote the article and reviewed and/or edited the

manuscript before submission. L.P. researched data for the arti-

cle, contributed substantially to discussion of the content and

reviewed and/or edited the manuscript before submission.