Beskæftigelsesudvalget 2018-19 (1. samling)
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Original article
doi:10.5271/sjweh.3728
Scand J Work Environ Health
2018;44(4):403-413
Night work and hypertensive disorders of pregnancy: a
national register-based cohort study
by
Hammer P, Flachs E, Specht I, Pinborg A, Petersen S, Larsen A,
Hougaard K, Hansen J, Hansen Å, Kolstad H, Garde A, Bonde JP
In this nationwide study, we investigated the association of
hypertensive disorders of pregnancy with different dimensions of
night work objectively assessed through payroll data. Our results
indicate that working consecutive night shifts during the first 20
pregnancy weeks increases the risk of hypertensive disorders by 41%,
which may be considered when providing recommendations on
organization of night work during pregnancy.
Affiliation:
Department of Occupational and Environmental Medicine,
Bispebjerg University Hospital, Bispebjerg Bakke 23, indgang 20F,
2400 Copenhagen, Denmark. [email protected]
Refers to the following texts of the Journal:
2006;32(6):413-528
2007;33(4):241-320 1989;15(6):0 2010;36(2):81-184
2013;39(4):321-426 2015;41(3):219-324 2017;43(1):1-96
Key terms:
circadian disruption; cohort study; gestational
hypertension; hypertension; hypertensive disorder; night work; payroll
data; preeclampsia; pregnancy; register-based cohort study; shift
work; shift worker; work schedule; working time
This article in PubMed:
www.ncbi.nlm.nih.gov/pubmed/29669140
Additional material
Please note that there is additional material available belonging to
this article on the
Scandinavian Journal of Work, Environment & Health
-website.
Print ISSN: 0355-3140 Electronic ISSN: 1795-990X Copyright (c) Scandinavian Journal of Work, Environment & Health
 BEU, Alm.del - 2018-19 (1. samling) - Bilag 50: Orientering om ny viden om gravide og natarbejde, fra beskæftigelsesministeren
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O
riginal article
Scand J Work Environ Health. 2018;44(4):403‒413.
doi:10.5271/sjweh.3728
Night work and hypertensive disorders of pregnancy: a national register-based cohort study
by Paula Hammer, MD,
1
Esben Flachs, MSc, PhD,
1
Ina Specht, MSc, PhD,
2
Anja Pinborg, MD, PhD,
3
Sesilje Petersen, MSc, PhD,
1
Ann Larsen, MSc, PhD,
4
Karin Hougaard, MSc, PhD,
4, 5
Johnni Hansen, MSc, PhD,
6
Åse Hansen, MSc, PhD,
4, 5
Henrik Kolstad,
PhD,
7
Anne Garde, PhD,
4, 5
Jens Peter Bonde, MD, PhD
1, 5
Hammer P, Flachs E, Specht I, Pinborg A, Petersen S, Larsen A, Hougaard K, Hansen J, Hansen Å, Kolstad H, Garde A, Bonde
JP. Night work and hypertensive disorders of pregnancy: a national register-based cohort study.
Scand J Work Environ Health
2018;44(4):403‒413.
doi:10.5271/sjweh.3728
Objective
The aim of this study was to investigate whether night work expressed by number and duration of night
shifts, number of consecutive night shifts, and number of quick returns during the first 20 weeks of pregnancy
is a risk factor for hypertensive disorders of pregnancy (HDP).
Methods
The study population comprised Danish workers in public administration and hospitals who gave birth
between 2007 and 2013. Exposure was assessed objectively through payroll data. Information on the outcome
was retrieved from the National Patient Register. We performed logistic regression on the risk for HDP accord-
ing to night work adjusted for age, body mass index (BMI), parity, socioeconomic status, and sickness absence
prior to pregnancy.
prevalence of HDP was 3.7%. Among night workers, the risk of HDP grew with increasing number of consecu-
tive night shifts [odds ratio (OR) 1.41, 95% confidence interval (CI) 1.01–1.98) and of quick returns after night
shifts (OR 1.28, 95% CI 0.87–1.95). Among obese women (body mass index ≥30 kg/m
2
), those who worked
long night shifts and longer spells of consecutive night shifts, and had the highest number of quick returns after
night shifts, had a 4–5 fold increased risk of HDP compared to day workers.
Results
Among 18 724 workers, 60% had at least one night shift during the first 20 weeks of pregnancy. The
Conclusion
Working consecutive night shifts and quick returns after night shifts during the first 20 pregnancy
weeks was associated with an increased risk of HDP, particularly among obese women.
Key terms
circadian disruption; gestational hypertension; hypertension; payroll data; preeclampsia; shift work;
shift worker; work schedule; working time.
Around 14% of the female European workers <50 years
engage in night work (1).
Several studies have investi-
gated adverse pregnancy outcomes in relation to work
schedules during pregnancy (2–6), but studies focusing
on the pregnant women’s health are sparse (7–9).
Hypertensive disorders of pregnancy (HDP) includ-
ing preeclampsia and gestational hypertension occur
in around 8% and 5% of pregnancies worldwide and
in Denmark, respectively, and are a major cause of
morbidity and mortality (10–12).
It is suggested that
the incidence of HDP has increased over time probably
due to advanced maternal age and increased occurrence
of obesity and diabetes in mothers (13,
14).
The patho-
physiology of HDP is not fully elucidated but seems to
involve maternal, fetal and placental factors (14–19).
Night work, including both fixed night shifts and
shift work, may influence the risk of HDP in several
ways. Psychosocial factors related to night work, such as
low job control and work-life conflict, have been associ-
ated with cardiovascular diseases including hypertension
(20,
21).
Another mechanism is through behavioral
changes induced by night work affecting sleep, smoking
1
2
3
4
5
6
7
Department of Occupational and Environmental Medicine, Bispebjerg University Hospital, Copenhagen, Denmark.
The Parker Institute, Bispebjerg and Frederiksberg University Hospital, Copenhagen, Denmark.
Department of Obstetrics and Gynecology, Hvidovre University Hospital, Copenhagen, Denmark.
National Research Centre for the Working Environment, Copenhagen, Denmark.
University of Copenhagen, Department of Public Health, Copenhagen, Denmark.
Danish Cancer Society Research Center, Copenhagen, Denmark.
Department of Occupational Medicine, Danish Ramazzini Centre, Aarhus University Hospital, Aarhus, Denmark.
Correspondence to: Paula EC Hammer, Bispebjerg Bakke 23, indgang 20F, 2400 Copenhagen, Denmark. [E-mail: [email protected]]
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Night work and hypertensive disorders of pregnancy
habits, physical activity, diet and body mass (20,
22).
Furthermore several physiological mechanisms includ-
ing circadian disruption, hormonal changes, altered
lipids and increased inflammation markers have been
proposed linking night work with cardiovascular dis-
eases (20,
22, 23). Melatonin, one of the main hormones
affected by circadian disruption, is also produced in the
placenta and plays a crucial role in maternal, fetal and
placental physiology acting as an anti-inflammatory and
immunomodulatory hormone, as well as a regulator of
apoptosis (24–32).
Furthermore the circadian oscilla-
tion of blood pressure is controlled in part by melatonin
(33,
34).
An altered circadian pattern of blood pressure
has been reported in HDP, and as a result melatonin
has been studied for its potential use in the treatment of
preeclampsia (35,
36).
The few studies that have been conducted on the
association between night and shift work with HDP
revealed conflicting results (37–40). A major limita-
tion of these studies is the crude assessment of work
schedules. For instance in three (37, 38, 40)
out of four
studies it was not clear whether their definition of shift
work included night shifts.
Payroll data provides accurate information on work
schedules for a large population overcoming hereby
the limitations related to exposure assessment in prior
studies (41,
42).
The primary aim of this study was to investigate
whether night work expressed by number and duration
of night shifts, number of consecutive night shifts and
number of quick returns during pregnancy is related
to increased risk of HDP. We furthermore investigated
whether age, body mass index (BMI) and socioeconomic
status (SES) modified the effect of night work on the
risk of HDP.
Medical Birth Registry, which contains information
from all home and hospital births in Denmark from 1973
onwards (44).
Outcome variables were identified from
the Danish National Patient Registry, which provides
data on inpatients in Danish hospitals since 1977 and
on outpatients since 1994 (45).
Study cohort
Women from DWHD who gave birth at least once
between 2007 and 2013 were identified (N=42 485
women with 60 482 births). We excluded women ≤18
and ≥50 years (N=15), multiple pregnancies (N=2957),
pregnancies conceived in 2006 (N=6403), and pregnan-
cies from women without registrations in DWHD of any
night or day shift during the first 20 pregnancy weeks
(N=26 481). This was the exposure time because gesta-
tional hypertension is, by definition, diagnosed after 20
pregnancy weeks (14). To avoid clustering effects, each
woman contributed with only one pregnancy, the first
during the study period (N=5902 pregnancies excluded),
leaving 18 724 women eligible for analyses (figure 1).
Exposure
Exposure definitions are in line with recent studies using
payroll data (41,
46).
Shifts, including on-call shifts, lasting ≥3 hours were
defined as day (start time after 06:00 and end time before
21:00 hours), evening (end time after 21:00 and before
02:00 hours), night (any start and end time including
working hours between 23:00 and 06:00 hours) and early
morning (start time between 03:00 and 06:00 hours).
A night worker was defined by working ≥1 night
shift and a day worker by working ≥1 day shift but no
night, evening or early morning shifts during the first 20
pregnancy weeks.
Consecutive night shifts.
Categories of consecutive night
shifts were 0 (only single night shifts), 2–3 (at least one
spell of 2–3 consecutive night shifts and no spells of ≥4
consecutive night shifts), and ≥4 (at least one spell of ≥4
consecutive night shifts) during the first 20 pregnancy
weeks.
Quick returns.
We defined quick returns as intervals
between shifts lasting <11 hours (47).
Quick returns after
night shifts were defined as a recovery period of <28 hours
after a night shift (46).
Categories of number of quick
returns and quick returns after night shift were 0, 1–4 and
≥5 quick returns during the first 20 pregnancy weeks.
Duration of night shifts.
Long night shift workers were
defined by working ≥1 long night shift (≥12 hours) dur-
ing the first 20 pregnancy weeks.
Methods
Design
We conducted a prospective register-based cohort study
with information from three Danish national registries
linked on individual level through the civil registration
number given to all residents in Denmark since 1968.
The Danish Working Hour Database (DWHD), a
national payroll database covering more than 250 000
employees in the Danish administrative regions includ-
ing all hospital employees, provided the source popula-
tion. It includes daily information on time of start and
end of all workdays, sickness absence, paid and unpaid
leave, occupation and place of employment from Janu-
ary 2007 to December 2015 (41, 43).
Pregnancy infor-
mation and covariates were identified from the Danish
404
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Hammer et al
Danish Working Hour Database:
265 702 public hospital employees
Figure 1.
Flowchart for identification of the study population.
Linkage with Medical Birth Registry
60 482 pregnancies from 42 485 women
who have given birth at least once
between January 2007 and December
2013
Age < 18 or > 50 years old
(N=15 pregnancies)
Multiple pregnancies
(N=2957)
Conception in 2006
(N=6403 pregnancies)
Not employed or any day or night
shift during the first 20 pregnancy
weeks (N=26 481 pregnancies)
Excluded
18 724 women eligible for analyses
Recurrent pregnancies (N=5902)
Number of night shifts.
Number of night shifts was analyzed
in categories of 1–19 or ≥20 (roughly corresponding to
≥1 night shift/week during the first 20 pregnancy weeks).
Covariates
Age (<30, 30–35, >35 years), BMI (<18.5, 18.5–24.9,
25–29.9, ≥30 kg/m
2
), parity (1, 2, ≥3) and smoking
(nonsmoker, former smoker, smoker) registered by the
midwife or family doctor at the first antenatal visit were
retrieved from the Danish Medical Birth Registry. Clas-
sification of SES into high, low or medium was derived
from Statistics Denmark. It was based on DISCO-88,
the Danish version of the International Standard Classi-
fication of Occupations (ISCO-88) (48),
in the calendar
years 2007–2009, and DISCO-08, the Danish version of
ISCO-08 (49), in the calendar years 2010–2013.
Sickness absence three months prior to pregnancy
was expressed as the sum of all days registered with ≥3
hours of sickness absence in DWHD during this period.
It was categorized as 0, <10 and ≥10 days.
Missing values for parity, smoking and SES repre-
sented only 1.4%, 2.9% and 0.2% respectively. Missing
values for BMI (4.4%) were evenly distributed across
exposure categories. Missing values of sickness absence
three months prior to conception (7.9%) occurred when
the woman’s employment covered by DWHD had <3
months prior to conception.
Outcome
The outcome of HDP was defined by ICD-10 codes (50):
hypertension (I10-15), gestational hypertension (O12,
13, 16) and pre-eclampsia and eclampsia (O14, 15).
Statistical analysis
We computed odds ratios (OR) with 95% confidence
intervals (CI) for HDP according to different dimensions
of night work during the first 20 weeks of pregnancy by
logistic regression. Model 1 refers to crude analyses and
model 2 is adjusted for age, BMI, parity, smoking, SES
and sickness absence three months prior to pregnancy
categorized as described above. Because of too few cases,
it was not possible to adjust the analyses for cases of
prior HDP (N=287), prior diabetes (N=17), and current
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Night work and hypertensive disorders of pregnancy
gestational diabetes (N=202). Model 3 is further adjusted
for number of night shifts in the analyses of consecutive
night shifts, quick returns and duration of night shifts.
In all analyses, except for interaction analyses, we
made comparisons of night workers with day workers
and comparisons within night workers. In the latter, night
workers in the lowest category of exposure (1–19 night
shifts, duration of night shift of <12 hours, night work-
ers without consecutive night shifts and night workers
without quick returns) were used as the reference group.
We investigated whether the association between
night work and HDP was modified by age, BMI and SES
by a likelihood ratio test comparing models with main
effects only with models that in addition included an
interaction term, ie, the product of the combined effect.
We used a level of significance of 5%.
Gestational length was used to identify conception
date. There were only 330 (0.6%) pregnancies with
missing values for gestational length but the proportion
of still births among these was statistically significant
higher (15.4%) than among other pregnancies (0.4%).
We therefore substituted the missing values by the mean
value of gestational length for live (278 days) and still
(220 days) births, respectively.
We performed the following sensitivity analyses: (i)
restricted to nulliparous women (N=9 660), (ii) with pre-
eclampsia as the outcome (N=18 724), and (iii) restricted
to the first trimester as the exposure time (N=18 158). In
the latter analysis, night workers had at ≥1 night shift
and day workers ≥1 day shift but no night, evening or
early morning shifts during the first 12 pregnancy weeks
instead of 20 weeks applied in the main analysis.
All analyses were done with the SAS 9.4 software
(SAS Institute, Cary, North Carolina, United States).
Results
In our cohort of 18 724 pregnant women, 11 193 were
classified as night workers and 7531 as day workers (table
1). The most frequent occupations were nurse (44%),
physician (13%), medical secretary (7%), physio/occu-
pational therapist (5%) and laboratory technician (4%)
reflecting that the majority of the workers covered by the
DWHD are employed at hospitals. Characteristics of day
Table 1.
Characteristics of pregnant workers in public administration and hospitals in Denmark, 2007–2013. [SD=standard deviation.]
Day work
a
(N=7531)
N
Age (years)
Body mass index (kg/m
2
)
Parity
1
2
≥3
Smoking
Non smoker
Former smoker
Smoker
Socioeconomic status
High
Medium
Low
Most frequent
c
occupations
Nurse
Physician
Nurse assistant
Laboratory technician
Midwife
Medical secretary
Physio- and ergo therapist
Cleaning and kitchen staff
Psychologist
Shifts during the first 20 weeks of pregnancy
Day
Night
Evening
Early morning
Weekly working hours
d
Sickness absence days 3 months prior to pregnancy
a
b
Night work
b
(N=11 193)
SD
4.2
7.6
N
%
Mean
30.7
23.9
SD
3.9
7.9
%
Mean
31.8
24.0
3462
2596
1356
6952
106
273
1844
3848
1821
1071
659
208
503
10
1314
977
391
463
46.0
34.5
18.0
92.3
1.4
3.6
24.5
51.1
24.2
14.2
8.8
2.8
6.7
0.1
17.5
13.0
5.2
6.2
65.9
23.8
6198
3156
1701
10 319
230
309
2618
7575
988
6,857
2,000
390
259
248
58
30
8
1
55.4
28.2
15.2
92.2
2.1
2.8
23.4
67.7
8.8
61.3
17.9
3.5
2.3
2.2
0.5
0.3
0.07
0.01
37.9
11.2
9.2
0.03
25.1
2.6
17.2
9.1
9.7
0.8
7.7
5.6
23.8
2.9
8.9
7.2
≥1 day shift and no night, evening or early morning shift during the first 20 pregnancy weeks.
≥1 night shift during the first 20 pregnancy weeks.
c
Occupations with ≥100 subjects.
d
Paid and unpaid leave excluded.
406
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Hammer et al
and night workers were rather similar. Day workers had
a total of 496 024 day shifts during the first 20 weeks of
pregnancy. Night workers had a total of 652 858 shifts
being 65% day shifts and 19% night shifts. Only 113
women (1%) worked fixed night shifts. They had higher
BMI (mean 25.2 kg/m
2
, SD 5.0), higher proportion of
women with parity ≥3 (27%), higher proportion of current
smokers (4.4%) and higher proportion of women with
low SES (43%) compared to the other night workers in
the cohort. The prevalence of HDP was 3.6% among day
workers and 3.8% among night workers.
Women working ≥1 spell of ≥4 consecutive night
shifts during the first 20 pregnancy weeks had higher
risk of HDP compared to night workers without con-
secutive night shifts (OR 1.41, 95% CI 1.01–1.98), see
table 2. We furthermore observed a statistically signifi-
cant trend of increasing risk with increasing number of
consecutive night shifts. Women with spells of exclu-
Table 2.
Odds ratios (OR) of hypertensive disorders of pregnancy by consecutive night shifts during the first 20 pregnancy weeks among workers in
public administration and hospitals in Denmark, 2007–2013. [CI=confidence interval]
Consecutive night shifts
N
Women
%
N
Cases
%
OR
Model 1
a
95% CI
OR
Model 2
b
95% CI
Referent
0.67–1.08
0.79–1.20
0.86–1.48
0.62
Referent
0.92–1.62
1.01–1.98
0.04
All workers (N=18 724)
Day work
c
7531
40.2
270
3.6
1.00
Referent
1.00
0
4003
21.4
132
3.3
0.92
0.74–1.13
0.85
2–3
5225
27.9
205
3.9
1.10
0.91–1.32
0.97
≥4
1965
10.5
89
4.5
1.28
0.99–1.62
1.13
P for trend
0.05
Night workers
d
(N=11 193)
0
4003
35.8
132
3.3
1.00
Referent
1.00
2–3
5225
46.7
205
3.9
1.20
0.96–1.50
1.22
≥4
1965
17.6
89
4.5
1.39
1.06–1.83
1.41
P for trend
0.02
a
Crude analysis.
b
Adjusted for categories of age, body mass index, smoking, socioeconomic status, parity and sickness absence three months prior to pregnancy.
c
≥1 day shift and no night, evening or early morning shift during the first 20 pregnancy weeks.
d
≥1 night shift during the first 20 pregnancy weeks.
Table 3.
Odds ratios (OR) of hypertensive disorders of pregnancy by number of quick returns
a
and quick returns after a night shift
b
during the first
20 pregnancy weeks among workers in public administration and hospitals in Denmark, 2007–2013. [CI=confidence interval]
Women
N
Quick returns
All workers (N=18 724)
Day work
e
0
1–4
≥5
P for trend
Night workers
f
(N=11 193)
0
1–4
≥5
P for trend
%
N
Cases
%
OR
Model 1
c
95% CI
OR
Model 2
d
95% CI
7531
3817
5123
2253
3817
5123
2253
40.2
20.4
27.4
12.0
34.1
45.8
20.1
270
128
203
95
128
203
95
3.6
3.4
4.0
4.2
3.4
4.0
4.2
1.00
0.93
1.11
1.18
0.10
1.00
1.19
1.27
0.07
Referent
0.75–1.15
0.92–1.34
0.93–1.50
Referent
0.95–1.49
0.97–1.66
1.00
0.92
1.00
0.94
0.76
1.00
1.12
1.07
0.64
Referent
0.72–1.16
0.81–1.23
0.72–1.22
Referent
0.87–1.45
0.79–1.46
Quick returns after a night shift
All workers (N=18 724)
7531
40.2
270
3.6
1.00
Referent
1.00
Day work
e
0
1023
5.5
39
3.8
1.07
0.75–1.48
0.84
1–4
4569
24.4
160
3.5
0.98
0.80–1.19
0.86
≥5
5601
29.9
227
4.1
1.14
0.95–1.36
1.06
P for trend
0.26
0.74
Night workers
f
(N=11 193)
0
1023
9.1
39
3.8
1.00
Referent
1.00
1–4
4569
40.8
160
3.5
0.92
0.65–1.33
1.03
≥5
5601
50.0
227
4.1
1.07
0.76–1.53
1.28
P for trend
0.30
0.05
a
<11 hours between two consecutive shifts.
b
<28 hours between a night shift and the consecutive shift.
c
Crude analysis.
d
Adjusted for categories of age, body mass index, smoking, socioeconomic status, parity and sickness absence three months prior to pregnancy.
e
≥1 day shift and no night, evening or early morning shift during the first 20 pregnancy weeks.
f
≥1 night shift during the first 20 pregnancy weeks.
Referent
0.55–1.23
0.69–1.07
0.87–1.29
Referent
0.69–1.59
0.87–1.95
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Night work and hypertensive disorders of pregnancy
Table 4.
Odds ratios (OR) of hypertensive disorders of pregnancy by duration of night shifts during the first 20 pregnancy weeks among workers in
public administration and hospitals in Denmark, 2007–2013. [CI=confidence interval]
Duration of night shifts
N
All workers (N=18 724)
Day work
c
<12 hours
d
≥12 hours
e
P for trend
Night workers
f
(N=11 193)
<12 hours
≥12 hours
a
b
Women
%
40.2
30.6
29.2
51.2
48.8
N
270
214
212
214
212
Cases
%
3.6
3.7
3.9
3.7
3.9
OR
1.00
1.04
1.09
0.37
1.00
1.04
Model 1
a
95% CI
Referent
0.87–1.25
0.90–1.31
Referent
0.86–1.27
OR
1.00
0.94
1.00
0.92
1.00
1.08
Model 2
b
95% CI
Referent
0.76–1.16
0.81–1.23
Referent
0.85–1.36
7531
5734
5459
5734
5459
Crude analysis.
Adjusted for categories of age, body mass index, smoking, socioeconomic status, parity and sickness absence three months prior to pregnancy.
c
≥1 day shift and no night, evening or early morning shift during the first 20 pregnancy weeks.
d
≥1 night of <12 hours and no night shifts of ≥12 hours.
e
≥1 night shift of ≥12 hours.
f
≥1 night shift during the first 20 pregnancy weeks.
Table 5.
Odds ratios (OR) of hypertensive disorders of pregnancy by number of night shifts during the first 20 pregnancy weeks among workers in
public administration and hospitals in Denmark, 2007–2013. [CI=confidence interval]
Number of night shifts
N
Women
%
N
Cases
%
Model 1
a
OR
95% CI
Model 2
b
OR
95% CI
Referent
0.78–1.12
0.81–1.45
Referent
0.86–1.52
All workers, N=18 724
Day work
c
7531
40.2
270
3.6
1.00
Referent
1.00
1–19
9560
51.1
360
3.8
1.05
0.90–1.24
0.94
≥20
1633
8.7
66
4.0
1.13
0.85–1.48
1.09
P for trend
0.35
0.96
Night workers
d
, N=11 193
1–19
9560
85.4
360
3.8
1.00
Referent
1.00
≥20
1633
14.6
66
4.0
1.08
0.82–1.40
1.15
a
Crude analysis.
b
Adjusted for categories of age, body mass index, smoking, socioeconomic status, parity and sickness absence three months prior to pregnancy.
c
≥1 day shift and no night, evening or early morning shift during the first 20 pregnancy weeks.
d
≥1 night shift during the first 20 pregnancy weeks.
sively 2–3 consecutive night shifts had, on average, 4
consecutive night shifts in total. While women with ≥1
spell of ≥4 consecutive night shifts had, on average, 14
consecutive night shifts in total. Hence these categories
express both length of spells and total number of con-
secutive night shifts.
As shown in
table 3,
we observed a statistically sig-
nificant trend of increasing risk of HDP with increasing
number of quick returns after night shifts. However,
the risk estimate for the highest exposed group, those
with ≥5 quick returns after a night shift (on average
10.4 quick returns) during the first 20 pregnancy weeks,
did not reach statistical significance (OR 1.28, 95% CI
0.87–1.95).
Table 4
presents the results for long-night-shift
workers compared to day workers (OR 1.00, 95% CI
0.81–1.23), and compared to short-night-shift workers
(OR 1.08, 95% CI 0.85–1.36). Of all long night shifts,
40% lasted 17–24 hours and 34% lasted 9–16 hours,
while 62% of all short night shifts lasted ≤8 hours.
Table 5
presents the results for women who worked
≥20 night shifts during the first 20 pregnancy weeks
(on average 28 night shifts) compared to day workers
(OR 1.13, 95% CI 0.85–1.48), and compared to women
working 1–19 night shifts (OR 1.15, 95% CI 0.86–1.52).
Further adjustment for number of night shifts (model
3) did not substantially change the results in the analyses
of consecutive night shifts, quick returns and duration
of night shifts.
The association between night work and HDP was
modified by BMI (P-value for multiplicative interaction
0.03). As presented in table 6,
analysis among women
with BMI ≥30 kg/m
2
revealed that those who worked
≥4 consecutive night shifts had substantially increased
risk of HPD compared to day workers (OR 5.31, 95%
CI 1.98–14.22). The corresponding risk for women
with BMI <25 kg/m
2
was OR 1.02, 95% CI 0.73–1.41.
Further adjustment for BMI among obese women did
not change the results. A similar increase was observed
for all exposures among obese women (see supple-
mentary tables S1-S4,
www.sjweh.fi/show_abstract.
php?abstract_id=3728).
Due to low statistical power
we were unable to make stratified comparisons within
night workers only. We found no interaction of any of
the analyzed exposures with maternal age or SES.
Overall sensitivity analyses slightly attenuated the
estimates across all exposures. The effect of consecutive
night shifts during the first 20 pregnancy weeks was con-
408
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Hammer et al
Table 6.
Odds ratios (OR) of hypertensive disorders of pregnancy by consecutive night shifts during the first 20 pregnancy weeks stratified
a
by body
mass index (BMI) among workers in public administration and hospitals in Denmark, 2007–2013. [CI=confidence interval]
Consecutive night shifts
N
2
Women
%
N
Cases
%
OR
Model1
b
95% CI
OR
1.00
0.77
0.95
1.02
1.00
0.93
0.97
0.94
1.00
3.47
1.60
5.31
Model2
c
95% CI
Referent
0.59–1.02
0.75–1.21
0.73–1.41
Referent
0.55–1.56
0.63–1.51
0.53–1.69
Referent
1.15–10.52
0.53–4.83
1.98–14.22
BMI <25 kg/m (N=12 815)
Day work
5119
40.0
201
3.9
1.00
Referent
0
2952
23.0
96
3.3
0.82
0.64–1.05
2–3
3545
27.7
154
4.3
1.11
0.90–1.38
≥4
1199
9.4
56
4.7
1.20
0.89–1.62
BMI 25–29 kg/m
2
( N=3501)
Day work
1419
40.5
52
3.7
1.00
Referent
0
644
18.4
24
3.7
1.02
0.62–1.67
2–3
994
28.4
40
4.0
1.10
0.72–1.68
≥4
444
12.7
16
3.6
0.98
0.56–1.74
BMI ≥ 30 kg/m
2
(N=1588)
Day work
671
42.3
7
1.0
1.00
Referent
0
229
14.4
10
4.4
4.33
1.63–11.52
2–3
435
27.4
8
1.8
1.78
0.64–4.94
≥4
253
15.9
13
5.1
5.14
2.03–13.03
a
P–value for multiplicative interaction 0.0317.
b
Crude analysis.
c
Adjusted for categories of age, smoking, socioeconomic status, parity and sickness absence three months prior to pregnancy.
sistent although not statistically significant throughout
sensitivity analyses within night workers (OR 1.39, 95%
CI 0.94–2.05 restricted to nulliparous women, OR 1.40,
95% CI 0.91–2.15 with pre-eclampsia as the outcome,
and OR 1.36, 95% CI 0.96–1.93 with the first trimester
as the exposure time). Regarding the question on pos-
sible selection out of night work during pregnancy, we
identified only 580 women (5%) who worked at least
one night shift during the first trimester and changed
to fixed day work during the second trimester. These
women had similar age (mean 31 years), BMI (mean
23.7 kg/m
2
) and smoking habits (2.8% current smokers)
as the rest of the cohort but presented a higher propor-
tion of physicians (37%).
Discussion
To our knowledge, this is the first study to investigate
the association between HDP with different dimensions
of night work objectively assessed through payroll data.
In our study, workers with ≥4 consecutive night shifts
during the first 20 pregnancy weeks had higher risk of
HDP compared to night workers without consecutive
night shifts (OR 1.41, 95% CI 1.01–1.98). We further-
more observed a dose−response gradient for number of
consecutive night shifts and the risk of HDP. The fact
that this effect was observed in comparisons within night
workers strengthens the evidence of a causal effect as
the group of night workers is more homogeneous. These
analyses may therefore be less susceptible to the healthy
worker effect present in comparisons of night versus
day workers. In fact, we observed higher risk estimates
in comparisons within night workers for all the expo-
sures. Comparisons within night workers may be more
appropriate from an epidemiological point of view. On
the other hand, analyses restricted to night workers
exclude an unexposed group and some selection bias
regarding different dimensions of night work remains.
Previous studies have shown that individual preferences
related to both personal (chronotype, sleep flexibility,
social context) (51–53) and occupational (work content,
demands and environment) (54) factors vary substan-
tially among night workers resulting in differences in
adaptation to night work. Accordingly we found that
workers with fixed night work during the first 20 preg-
nancy weeks differed in BMI, parity, smoking habits and
SES compared to the other night workers in the cohort.
Compared to the background Danish population, our
cohort presented lower prevalence of smoking during
pregnancy (3% versus 12%) (55)
and lower proportion
of overweight women (19% versus 46%) (56),
which
may reflect a more health promoting behavior among
healthcare professionals.
Our findings are in accordance with recent studies
focusing on consecutive night shifts rather than solely
on the number of night shifts. For example increasing
the number of consecutive night shifts has been associ-
ated with progressive changes in hormones involved
in circadian regulation, such as melatonin, cortisol,
thyroxin and prolactin (30,
31, 57). Such changes have
been observed down to three consecutive night shifts
(58,
59). Furthermore, it has been suggested that at least
two days off work are required to allow for circadian
readjustment following 2–4 consecutive night shifts (31,
60). In our cohort, the majority of hospital employees
had rotating shifts with different schedules nearly every
week which do not fulfill this recommendation. Hence,
in this context, working consecutive night shifts may
Scand J Work Environ Health 2018, vol 44, no 4
409
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1960725_0009.png
Night work and hypertensive disorders of pregnancy
lead both to circadian disruption and to insufficient
recovery. Our findings of increasing odds ratios of HDP
with increasing number of quick returns after night shifts
also support the potential effect of insufficient recovery
after a night shift.
In our data, BMI modified the effect of night work
on the risk of HDP, as obese women who worked longer
night shifts, longer spells of consecutive night shifts
and had the highest number of quick returns after night
shifts had 4–5 fold increased risk of HDP compared to
day workers. It is known that pre-pregnancy BMI is an
important risk factor for HDP independent of weight
gain during pregnancy (17,
61, 62). Even though these
results are based on few cases, they are consistent across
exposures. Obese women neither had higher propor-
tion of workers with fixed night shifts nor a gradient of
increasing BMI from day to night workers.
We hypothesized that women who worked night
shifts during the first trimester and changed working
schedule to only day work during the second trimester
due to health problems might cause bias towards the null
as the exposure time in the main analysis was 20 weeks.
However, sensitivity analysis resetting exposure time to
the first 12 pregnancy weeks indicated no such bias. On
the other hand, analysis restricted to the first trimester
excludes a possible effect of night work during the sec-
ond trimester, which may in part explain the attenuation
of the estimates. Even though the physiopathology of
HDP seems to be related with placenta development
in the beginning of pregnancy (14), demographic and
lifestyle factors on the second and third trimester of
pregnancy seem also to influence the risk of HDP (63).
We found no statistically significant association
between HDP with any of the analyzed dimensions of
night work compared to day workers, suggesting that
the effect of night work on the risk of HDP is related to
the way night shifts are organized rather than the mere
presence of night shifts. This can be in part due to dif-
ferences in work content and work environment between
day and night workers. We did not observe the presence
of more pronounced risk factors
for HDP among night
workers compared to day workers. Actually our cohort
of night workers had lower BMI, a lower proportion of
smokers and a lower proportion of workers with low
SES than day workers. Similar to
our results, three out
of four previous studies that compared shift workers
with day workers found no association between with
HDP (37,
39, 40). Wergeland & Strand (38) reported
an increased prevalence of pre-eclampsia among shift
workers, but only among parous women.
The main strengths of our study are the large and
national sample size, the objective and detailed expo-
sure assessment, and the use of validated and objective
registries for identification of covariates and outcomes,
which makes information bias and selection in and out
of the study unlikely. Furthermore, we evaluated differ-
ent dimensions of night work within night workers and
restricted the exposure time to specific periods of preg-
nancy. Some limitations include a lack of information
on workload during night shifts, such as the possibility
for sleep during on call shifts, and on chronotype and
personal preferences of the participants. The latter is
especially relevant because night work is compulsory for
the majority of occupations in our cohort. Additionally,
our study design did not account for the healthy worker
effect, where women with health problems in general
tend to choose day work. As our cohort comprises
primarily healthcare professionals, our results may not
apply for pregnant workers in other occupations.
Ideally future studies on health effects of night work
during pregnancy should combine objectively assessed
work schedules with information on chronotype and
personal preferences, work content and environment,
and should perform comparisons both with day workers
and within groups of night workers.
Concluding remarks
In this nationwide study of Danish pregnant workers in
the public health sector with objectively assessed work
schedules, working consecutive night shifts and quick
returns after night shifts during the first 20 pregnancy
weeks was associated with an increased risk of HDP,
in particular among obese women. Possible ways for
avoiding such risk when organizing night work dur-
ing pregnancy are favoring single night shifts or short
spells of consecutive night shifts and reducing quick
returns by allowing for adequate recovery time follow-
ing night shifts.
Acknowledgments
This work was supported by The Danish Working Envi-
ronment Research Fund grant 31-2015-03 2015001705.
The establishment of the DWHD has been financed
by research grants from The Danish Working Environ-
ment Research Fund (23-2012-09), The Nordic Program
on Health and Welfare – Nordforsk (74809) and The
National Research Centre for the Working Environ-
ment. The Danish administrative regions have partially
financed the transfer of data to the cohort.
The Danish administrative regions are acknowledged
for the participation and willingness to provide data to
the DWHD. Jens Worm Begtrup, Lisbeth Nielsen and
Anders Ørberg are thanked for valuable work with data
management.
The authors declare no conflicts of interest.
410
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Hammer et al
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