Shift and night work during pregnancy and preterm birth—a cohort study of Swedish health care employees

This a prospective cohort study of pregnancies among health care professionals employed by Region Stockholm, from selected professions often working on shift and/or nights, that is nurses including midwives, nursing assistants and other related professions (e.g. accommodation assistants, careers, personal assistants). Physicians were not included in the present analysis due to less detailed information on working hours and night work.

Ethical permission for the study was granted by the Ethical Review Board in Stockholm (2016/2490–31; 2017/1157–32). Informed consent for participation in the study is not requested for register-based research.

The Swedish Medical Birth Register (MBR) provided data on outcome variables and some potential confounders (see below) that were collected at the initial interview in the first trimester at the prenatal care unit, typically at Week 10, and throughout the pregnancy and at childbirth, for all women and their children.The MBR was founded in 1973 and covers about 99% of all births in Sweden.It is one of the most complete birth registers in the world and has previously been validated. Its registered information on gestational age is considered to be reliable and quality is controlled on an annual basis. ^{135 135 135}

We identified all births (= 15 765) from the MBR concerning women who were employed any time between 2008 and 2017 and gave birth after the first day of employment. Out of these, 10 795 births were excluded due to any of the exclusion criteria: multiple births,= 2067, professionals other than those of interest for the study,= 2980, and births from women who were not employed during the whole pregnancy period,= 5122, as well as births from women without work registration or undefined working hours,= 626. Consequently, the present study included all singleton births (= 4970) and their mothers (= 3940) who were employed during their whole pregnancy period and had worked during at least one trimester of pregnancy at any time between 2008 and 2016. The reason for excluding women who terminated their employment during pregnancy was that their working hours could not be traced for the full pregnancy. n n n n n n n

Information on working hours was retrieved from the employee register HEROMA used by Region Stockholm. The HEROMA contains detailed personal information on working hours day-by-day (number of minutes worked each hour) for all employees employed from 2008 and onwards, including information on occupation and workplace. Data for working time were aggregated from HEROMA to define work shifts for each trimester at each included pregnancy. A work shift should have a duration of at least 4 h to be called a shift. Three types of shifts were identified: day shift (starts after 06:00 and ends no later than 18:00); afternoon shift (starts after 12:00 and ends later than 18:00, but not a night shift); and night shift (at least 3 h of work within 22:00–06:00). ¹³⁵

For each trimester, we classified type of work schedules as ‘always day work’, ‘day and/or afternoon work (no nights)’, ‘day and/or afternoon work, and night work’, and ‘night work only’; and night work as ‘no night work’ and ‘night work ever’ (also including those who worked nights only) per trimester. Frequency of night shifts was categorized as ‘no night work’, ‘1–10 times’, ‘11–25 times’ and ‘>25 times’; consecutive night shifts as ‘1–4 times’, ‘5–8 times’ and ‘>8 times’, and the average number of nights in a row as ‘1–1.5 nights’, ‘1.6–2.5 nights’ and ‘>2.5 nights’ per trimester.

We defined quick returns from night shifts as a recovery period of <28 h after a night shift, and quick returns from other shifts as a recovery period of <11 h after any other shift, in line with recent studies using payroll data.Categories of quick returns were ‘1–8 times’, ‘9–18 times’ and ‘>18 times’ per trimester. Long working for any shift (≥10 h) was classified as ‘no’, ‘1–10 times’, ‘11–20 times’ and ‘>20 times’, and long working weeks (working >40 h) as ‘no’, and ‘long week ever’ per trimester. ^{135 135} ,

The outcome of PTB was available through the MBR and was defined by a gestational length of <37 completed weeks.All the births, including stillbirths (= 17) occurring at 22 completed weeks of pregnancy or later were considered for the study. The gestational length is most often defined by ultrasound early in pregnancy, preferably at 11–14 weeks, based on crown-rump and biparietal diameter. ^{135 135} n

Demographic variables/covariates as potential confounders were identified based on a review of previous studies on risk factors for PTB.The demographic variables were mother’s age at childbirth, height, body mass index (BMI), parity and smoking habits at an initial interview at the prenatal care facilities at each pregnancy, obtained through the MBR. The highest level of the mother’s education at each included pregnancy was obtained from the longitudinal integration database for health insurance and labour market studies (LISA) at Statistics Sweden, used as a proxy for socioeconomic factors, and mother’s country of birth as a proxy measure for ethnicity categorized as Sweden, Nordic countries (except Sweden), Europe (except Nordic countries) and other countries. ¹³⁵

Baseline characteristics are presented as means (standard deviation: SD) for continuous variables and as percentages for categorical variables. Binary logistic regressions were used to estimate odds ratios (OR) with 95% confidence intervals (CI) for PTB in relation to the different dimensions of shift work and long working hours. Analyses were adjusted for the potential confounders. Some analyses were also adjusted for the number of total night shifts among those who worked at least one night shift during the trimester, in order to disentangle the effect of working consecutive night shifts or quick returns from the intensity of night work in general. We performed comparisons of different dimensions of night shift work with non-night shift work, and comparisons within those with at least one night shift during any trimester where the night shift workers with the lowest exposure category served as the reference group. The latter was done to strengthen the evidence of a causal effect, as the category of night shift workers is more homogeneous and may be less susceptible to systematic selection from night work. To capture uncontrolled confounding by any previous negative birth experience, we also applied sensitivity analyses. First, the analysis was delimited to first-time pregnant women. Secondly, we investigated if a PTB at first pregnancy influenced night work during second or later pregnancies, by comparing the average number of night shifts in mothers with the first child born full-term with the average number of night shifts in mothers whose first child was born preterm.

The statistical analyses were conducted using SAS software, version 9.4 for Windows (SAS Institute Inc., Cary, NC, USA), with the statistical association at a two-tailedvalue below 0.05. P

Women who ever worked the night shift during the first trimester had a slightly increased risk of PTB [adjusted OR (aOR) 1.32; 95% CI, 0.98–1.76], compared with no night work. Women who worked the night shift frequently (>25 times) during the first trimester were 1.62 times more likely to give birth to an infant with PTB (aOR, 1.62; 95% CI, 1.03–2.53) compared with those with no night shifts. No associations were observed in the analysis of the second and third trimesters, except for working night shifts 1–10 times during the third trimester (aOR, 1.57; 95% CI, 1.03–2.39); (). Among first-time mothers, associations were present also for trimesters two and three, see below. Table 2

Women who ever worked three or more consecutive night shifts during the first trimester were 43% more likely to give birth to an infant with PTB (aOR, 1.43; 95% CI, 1.03–1.99); compared with those with no night work during that trimester. The risk for PTB was higher (aOR, 3.01; 95% CI, 1.38–6.55) among those who frequently (>8 times) worked three or more consecutive night shifts during the first trimester, compared with those who did so 1–4 times. Working on the average 1.6–2.5 nights and >2.5 nights in a row during the first trimester was associated with a 3–4-fold increased risk of PTB compared with working 1–1.5 nights in a row. After adjusting for the total number of night shifts, the associations remained for three or more consecutive night shifts and for the average number of nights during the first trimester, although with wider confidence intervals (). Table 2

Women who frequently (>18 times) had quick returns from night shifts during the first trimester had an increased risk for PTB (aOR, 2.42; 95% CI, 1.23–4.75), compared with those with quick returns 1–8 times. After adjusting for the total number of night shifts, more frequent quick returns from night shifts (9–18 times, and >18 times) during the first trimester was associated with PTB, compared with quick returns 1–8 times (). Increased aORs were observed for PTB in women working long shift (≥10 h) frequently (>20 times) during the first trimester (aOR, 1.63; 95% CI, 1.07–2.49), and working 1–10 times during the third trimester (aOR, 1.66; 95% CI, 1.16–2.35) compared with those with no longer shifts. Moreover, working any week for >40 h during the third trimester was associated with a more than 2-fold increased risk of PTB (aOR, 2.05; 95% CI, 1.31–3.22) (). Table 3 Table 3

There was a decrease in mean working hours per week (from 38.1 to 32.9 h) from the first month to the ninth month of gestation (). Although there was no remarkable decrease in the mean number of all shifts (from 17.5 to 17. 4 shifts), there was a decrease in the mean ratio of night shift work (from 30% to 19%) at the first month gestation to the ninth month of gestation (). Thus, there was a gradual transition to day work during pregnancy, and this transition started already in trimester two (). A preterm birth at first pregnancy was consistently associated with less night work at later pregnancies, both at the beginning and during the course of pregnancy (). In the first month of the second or later pregnancies, women with a first child born full term worked on the average 1.6 nights per month, whereas mothers with a first child born preterm worked 1.4 nights, and this difference persisted throughout pregnancy. Both the difference in the average number of night shifts and the ratio between night shifts in the two groups tended to increase during the course of pregnancy. The difference increased by 6.2% per month of gestation in a linear regression model (beta = 0.062,=0.10). The ratio of the average number of night shifts per month among mothers with and without previous PTB decreased from around 0.9 in the first month by a factor of 0.03 per gestational month, in a linear regression model (beta = 0.03,=0.08) (). Figure 1 Figure 2 Figure 2 Figure 3 Figure 3 P P

Sensitivity analyses of first-time pregnant women (= 2056) yielded broadly similar results as the main analyses for trimester one, although with a wider confidence interval. One important difference was that many aspects of night work were associated with increased risks also in trimesters two and three, and some risk associations in trimester one were slightly higher among the primiparous women than among all pregnant women. For example, working ever three or more consecutive nights during the second and/or third trimester were associated with PTB (aOR, 1.60; 95% CI, 1.01–2.54 and aOR, 1.71; 95% CI, 1.04–2.82, respectively) (, available asatonline), which was not the case in the main analyses. n IJE Supplementary Table S1 Supplementary data

Shift work, including night work and long working hours, during pregnancy is an exposure of concern according to the occupational safety regulations for pregnant women in the EU. The employer is obliged to make a risk assessment, but this is hampered by a lack of knowledge of which specific conditions in long work or night work constitute a risk.Thus, our study will contribute to better understand how to organize shift work during pregnancy to avoid or to minimize the risk. One strength of our study is that it includes women in the health care sector where the percentages of shift and night workers are high. Thus, our study will contribute to better understanding of the challenges and to developing solutions on how the health sector can offer better quality employment to women. Our study will guide us to find preventive measures against negative pregnancy outcomes and will contribute to policy making regarding working hours during pregnancy. ¹³⁵

The main strengths of our study include using a large and prospective cohort dataset with registered-based exposure information in an employee register. The data provided a unique opportunity to study shift work in more detail with several dimensions of the working hours or schedule specific to each trimester. All exposure information was collected before the outcome information was known. The outcome and confounders from the MBR and LISA are of high quality, with few missing observations. Information on the outcome, exposures and confounders was obtained at each pregnancy. With prospectively collected data, recall bias does not constitute a problem in our study. Another strength is that our study population represents a homogeneous group of health care employees with less variability in potential confounders related to socioeconomic position.

Our study had some limitations. We did not have information on physically demanding workload (e.g. heavy lifting, prolonged standing) during pregnancy, which was found to be associated with PTB in previous studies.However, the adjustment for occupation should have tended to decrease this source of error. Likewise, we had no information on exposure to anaesthetics or chemotherapeutics, but both these exposures would tend to be more common among day-time than among night-time workers. We did not have information on the sleep, chronotype and personal preferences of the participants. It was earlier found that women with morning chronotype tend to find night shifts more strenuous compared with women with evening chronotype.Our study includes only health care employees, with the majority having irregularly changing schedules nearly weekly. Therefore, our results may not apply to populations with other occupations. ^{135 135}