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Original article
Employment status and the risk of pregnancy complications: the Generation R Study
  1. Pauline W Jansen1,2,3,
  2. Henning Tiemeier3,4,
  3. Frank C Verhulst3,
  4. Alex Burdorf2,
  5. Vincent W V Jaddoe1,4,5,
  6. Albert Hofman4,
  7. Henriëtte A Moll5,
  8. Bero O Verburg6,
  9. Eric A P Steegers6,
  10. Johan P Mackenbach2,
  11. Hein Raat2
  1. 1The Generation R Study Group, Erasmus MC-University Medical Center Rotterdam, Rotterdam, the Netherlands
  2. 2Department of Public Health, Erasmus MC-University Medical Center Rotterdam, Rotterdam, the Netherlands
  3. 3Department of Child and Adolescent Psychiatry, Erasmus MC-University Medical Center Rotterdam, Rotterdam, the Netherlands
  4. 4Department of Epidemiology, Erasmus MC-University Medical Center Rotterdam, Rotterdam, the Netherlands
  5. 5Department of Paediatrics, Erasmus MC-University Medical Center Rotterdam, Rotterdam, the Netherlands
  6. 6Department of Obstetrics and Gynaecology, Division of Obstetrics and Prenatal Medicine, Erasmus MC-University Medical Center Rotterdam, Rotterdam, the Netherlands
  1. Correspondence to Pauline W Jansen, The Generation R Study Group, Erasmus MC-University Medical Centre Rotterdam, PO Box 2040 (Room Ae-029), 3000 CA Rotterdam, the Netherlands; p.w.jansen{at}erasmusmc.nl

Abstract

Objectives This study explored the relationships of employment status, type of unemployment and number of weekly working hours, with a wide range of pregnancy outcomes.

Methods Information on employment characteristics and pregnancy outcomes was available for 6111 pregnant women enrolled in a population-based cohort study in the Netherlands.

Results After adjustment for confounders, there were no statistically significant differences in risks of pregnancy complications between employed and unemployed women. Among unemployed women, women receiving disability benefit had an increased risk of preterm ruptured membranes (OR 3.16, 95% CI 1.49 to 6.70), elective caesarean section (OR 2.98, 95% CI 1.21 to 7.34) and preterm birth (OR 2.64, 95% CI 1.32 to 5.28) compared to housewives. Offspring of students and women receiving disability benefit had a significantly lower mean birth weight than offspring of housewives (difference: −93, 95% CI −174 to −12; and −97, 95% CI −190 to −5, respectively). In employed women, long working hours (≥40 h/week) were associated with a decrease of 45 g in offspring's mean birth weight (adjusted analysis; 95% CI −89 to −1) compared with 1–24 h/weekly working hours.

Conclusions We found no indications that paid employment during pregnancy effects the health of the mother and child. However, among unemployed and employed women, women receiving disability benefit, students and women with long working hours during pregnancy were at risk for some adverse pregnancy outcomes. More research is needed to replicate these results and explain these findings. Meanwhile, prenatal care providers should be made aware of the risks associated with specific types of unemployment and long working hours.

  • Employment
  • working hours
  • healthy worker effect
  • pregnancy
  • delivery
  • epidemiology
  • public health
  • female reproductive effects and adverse pregnancy outcomes

https://doi.org/10.1136/oem.2009.046300

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    Introduction

    The employment rate of women is increasing in many Western countries. In the European Union the proportion of employed women increased from about 50% in 1997 to 60% in 2007,1 implying that a growing number of women work during pregnancy. Studies in the general population show that paid employment is associated with better health status as compared to being unemployed,2–4 a phenomenon called the ‘healthy worker effect’.5 This raises the question whether there is also a positive association between paid employment and pregnancy outcomes, or whether there are unfavourable effects of paid employment regarding pregnancy outcomes due to, for example, overload.

    Earlier research on employment status during pregnancy was primarily aimed at exploring the relationship between paid employment (present versus absent) with gestational duration and birth weight; however, due to inconsistent findings the direction and magnitude of this association remains unclear.6–11 Whether paid employment is related to other indicators of neonatal health, maternal pregnancy complications and/or delivery characteristics remains largely unknown as research in this area is lacking.

    In addition to research comparing the pregnancy outcomes of employed and unemployed women, studies have also investigated the effect of long weekly working hours on pregnancy outcomes among employed women. It was consistently reported that there is no association between the number of weekly working hours and the risk of low birth weight8 9 12 13; however, for preterm birth inconsistent findings were reported.8–11 13 14 The relationship between weekly working hours and pregnancy outcomes other than gestational duration and birth weight has seldom been explored. A study in the USA indicated that pregnant women with gestational hypertension worked more hours per week than a control group,15 while Hung and colleagues found no association between the weekly working hours of pregnant women and the risk of caesarean delivery.16 Although several studies focused on employment characteristics (eg, the working hours of employed women), unemployed women as a group have seldom been examined in more detail. Therefore, it is still not known whether all unemployed women have the same risk of pregnancy complications or whether they are a more heterogeneous group.

    The present study examined the relationship between employment status during pregnancy and a wide range of pregnancy outcomes, such as maternal pregnancy complications, delivery outcomes and markers of neonatal health. Among unemployed pregnant women, we explored whether specific types of unemployment (housewife, job-seeking/receiving basic social security benefit, receiving disability benefit, and student) are associated with pregnancy outcomes. For employed pregnant women, the relationship between the number of weekly working hours and pregnancy outcomes was examined. For this, we used data from the Generation R Study, a large prospective birth-cohort study in the Netherlands.

    Methods

    Design

    This study was embedded in the Generation R Study, a population-based cohort study from fetal life onwards; this is described in detail elsewhere.17 18 All pregnant women living in the study area (Rotterdam, the Netherlands) were informed about the study by healthcare workers (eg, community midwives and obstetricians). In total, 8880 pregnant women of different ethnicities with a delivery date between April 2002 and January 2006 enrolled in the prenatal section of the study. Overall, 61% of all pregnant women in the study area who had a live birth participated in the Generation R Study (based on cohort years 2003 and 2004: 5189 of the 8494 live births participated). Assessments, including physical examinations, fetal ultrasound examinations and questionnaires, were planned in early (5–18 weeks' gestation; median 13 weeks), mid- (18–25 weeks' gestation; median 20 weeks) and late pregnancy (25–39 weeks' gestation; median 30 weeks). Written informed consent was obtained from all participants. The local Medical Ethical Committee approved the study.

    Population for analysis

    Of the 8880 participants who enrolled in the Generation R Study during pregnancy, those who had had a miscarriage (n=78), induced abortion (n=29), twin pregnancy (n=93), and those who enrolled in the study after 25 weeks' gestation (n=350) were excluded from the present study. Additionally, we excluded women without information on employment status (n=2219), yielding a sample size of 6111 pregnant women for the present analyses. Because data on one or more of the pregnancy complications were missing for some of the participants, the population for analyses varied per outcome (n between 5585 and 6110).

    Employment status and weekly working hours

    Information on employment during pregnancy was obtained by postal questionnaire in late pregnancy (≥25 weeks' gestation). The women were asked to indicate which of the following descriptions best applied to them at that moment: paid employment, self-employed, job-seeking/receiving basic social security benefit, receiving disability benefit, housewife or student. We generated the dichotomous variable ‘employment status’ by combining the first two categories (‘paid employment’) and merging the remaining categories (‘unemployed’). The number of weekly working hours of the participants with paid employment was assessed by means of the open question, ‘How many hours per week do you work?’. Working hours were categorised into ‘1–24’, ‘25–39’ and ‘40 or more hours a week’.

    Pregnancy complications

    Information on the following pregnancy complications was obtained from the medical records and delivery reports of the hospital and midwife registries.

    • Pregnancy-induced hypertension and pre-eclampsia were defined according to the criteria described by the International Society for the Study of Hypertension in Pregnancy.19 Pregnancy-induced hypertension was diagnosed if previously normotensive women had a systolic blood pressure of at least 140 mm Hg and/or a diastolic blood pressure of at least 90 mm Hg after 20 weeks' gestation; if pregnant women additionally had proteinuria (≥300 mg/24 h), then they were diagnosed as pre-eclamptic.

    • Gestational diabetes was diagnosed according to Dutch midwifery and obstetric guidelines using the following criteria: random glucose level above 11.1 mmol/l or a glucose level higher than 7.0 mmol/l after fasting, without previously diagnosed diabetes.

    • Preterm ruptured membranes before 37 weeks' gestation were defined as present (‘yes’) or absent (‘no’).

    • Poor progress of delivery was defined as failure to progress during the first and/or second stage of labour (yes/no).

    • Mode of delivery was categorised as ‘spontaneous delivery’, ‘assisted vaginal delivery’, ‘elective caesarean section’ and ‘emergency caesarean section’.

    • We determined gestational age by fetal ultrasound examination at the first visit to our research centre. Pregnancy dating curves were constructed using subjects for whom we had both ultrasound examinations at <25 weeks' gestation and reliable information on last menstrual period.20 Subsequently, all pregnancies in our study were dated using these curves. Birth was classified as preterm if it occurred at <37 weeks' gestation.

    • Meconium-stained amniotic fluid was defined as present (‘yes’) or absent (‘no’).

    • We dichotomised the continuous Apgar score at 5 min into <7 or ≥7.

    • Birth weight was measured in grams.

    • Small-for-gestational-age (SGA) was based on gestational age and gender specific birthweight distributions. Babies were defined as SGA if they were below the 10th percentile.21

    Confounders

    The following variables were considered as possible confounders in the association between employment characteristics and pregnancy outcomes. Age, height and weight of the pregnant women were assessed at enrolment. Body mass index (BMI) was calculated (kg/m2). A questionnaire was used to obtain information on ethnicity (Dutch, other Western, non-Western), educational level (low, high), family income (<€2000 and >€2000 per month), marital status (married/cohabiting, single), smoking and alcohol consumption during pregnancy, and parity. Smoking was categorised as: no smoking, <5 cigarettes per day and ≥5 cigarettes per day. Alcohol consumption was categorised according to the amount of alcoholic consumptions per week: no drinking, <1 drink per week, 1–6 drinks per week and ≥1 drinks per day. Due to the small numbers, the latter two categories were merged in the multivariate analyses. We defined parity as the number of live births the participants had previously delivered (0, ≥1). Maternal psychopathology was assessed using the Brief Symptom Inventory, a validated self-report22; the weighted sum score of the 53 items indicates the global severity of psychopathological symptoms, with higher scores denoting more symptoms. Information on the gender of offspring was obtained from the hospital and midwife registries.

    Statistical analyses

    Univariate logistic regression was used to calculate ORs for pregnancy complications among employed participants as compared to the reference group of unemployed pregnant women. Subsequently, in multivariate logistic regression analyses the ORs were adjusted for potential confounding factors. For birth weight (a continuous outcome) linear regression was applied; the multivariate linear regression analysis was also adjusted for gestational age at birth. Next, in the subgroup of unemployed women, we calculated adjusted ORs for pregnancy complications per subgroup of unemployment (job seeking, receiving disability benefit, student) as compared to the reference group of housewives. Finally, among participants with paid employment during pregnancy, logistic regression was applied to examine the adjusted association between weekly working hours and pregnancy outcomes; again, linear regression was applied for birth weight. To test for trends, we replicated the latter analyses including weekly working hours as a continuous variable. In the multivariate analyses, missing values on confounders were replaced by the median (categorical variables, BMI and psychopathological symptoms) or the mean (age). All statistical analyses were performed using SPSS v 11.0 for Windows.

    Non-response analyses

    Women with missing data on employment status (n=2219) were compared with women who filled out the questions on employment (n=6111). Data on employment status were relatively more often missing in pregnant women who were younger (F=67; df=1; p<0.001), had lower education (χ2=398; df=1; p<0.001), were of non-Western origin (χ2=296; df=1; p<0.001) and were single (χ2=160; df=1; p<0.001), and in those who smoked (χ2=14; df=1; p<0.001) as compared to women who filled out the questions on employment. Women with missing data more often had a preterm birth (χ2=8; df=1; p=0.004), an Apgar score below 7 at 5 min (χ2=8; df=1; p=0.004) and a lower birth weight of their offspring (F=63; df=1; p<0.001). Women with and without data on employment status did not differ with regard to the prevalence of pre-eclampsia (χ2=1.3; df=1; p=0.259), meconium-stained amniotic fluid (χ2=0,4; df=1; p=0.529) and caesarean section (elective: χ2=2.4; df=1; p=0.122; emergency: χ2=0.2; df=1; p=0.679).

    Results

    The characteristics of the pregnant women according to their employment status are presented in table 1. Of all participants, 28% (n=1703) were unemployed and 72% (n=4408) reported having paid work. The unemployed women were younger (F=598; df=1; p<0.001), had lower education (χ2=650; df=1; p<0.001) and were more often of non-Western origin (χ2=814; df=1; p<0.001) compared with women with paid employment. Unemployed women were also more often single (χ2=229; df=1; p<0.001) and reported more psychopathological symptoms (χ2=329; df=1; p<0.001). Employed women were more often nulliparous than unemployed women (χ2=116; df=1; p<0.001).

    View this table:
    Table 1

    Characteristics of the pregnant women according to their employment status

    Several different pregnancy complications were studied. Of all 6111 women, 3129 had no complications, 1546 had one complication, 1050 had two complications and 286 had three or more complications. Table 2 presents the association between the employment status of the participants and pregnancy complications. None of these associations remained statistically significant after adjustment for the confounders. The unadjusted decreased risks of preterm ruptured membranes (OR 0.71, 95% CI 0.54 to 0.95), preterm birth (OR 0.73, 95% CI 0.57 to 0.94), meconium-stained amniotic fluid (OR 0.85, 95% CI 0.73 to 0.99) and SGA (OR 0.78, 95% CI 0.65 to 0.93) among employed women attenuated to statistical non-significance in the adjusted analyses due to a combination of confounders, mainly family income, educational level, ethnicity, marital status and psychopathology of the women (data not shown). The unadjusted elevated risks of poor progress of delivery (OR 1.49, 95% CI 1.26 to 1.76) and non-spontaneous delivery (assisted vaginal delivery: OR 2.04, 95% CI 1.71 to 2.44; elective caesarean delivery: OR 1.72, 95% CI 1.27 to 2.33; emergency caesarean delivery: OR 1.26, 95% CI 1.01 to 1.58) in the employed group were also no longer statistically significant in the adjusted analyses; this was largely due to the primiparity of these women.

    View this table:
    Table 2

    Association between employment status of pregnant women and pregnancy outcomes

    Several women (n=159) indicated that they had stopped working during pregnancy due to pregnancy-related problems. Of these women, 84 were classified as unemployed and 75 as employed. To estimate whether this has influenced our results, we excluded these 159 women and then repeated the analyses on the association between employment status and pregnancy complications. The ORs and levels of statistical significance were similar before and after exclusion of women who had stopped working during pregnancy, indicating that this specific group did not bias our results.

    Table 3 shows the adjusted ORs for adverse pregnancy outcomes per type of unemployment as compared to paid employment. Housewives and job-seeking women had the same risk of adverse pregnancy outcomes as employed women. Women receiving disability benefit had an elevated risk of pregnancy induced hypertension (OR 2.22, 95% CI 1.00 to 4.91) and preterm birth (OR 2.35, 95% CI 1.28 to 4.31) as compared to employed women. Offspring of women receiving disability benefit and of students had a fourfold increased risk of an Apgar score below 7 at 5 min after birth (OR 4.19, 95% CI 1.48 to 11.9; and OR 3.71, 95% CI 1.36 to 10.1, respectively).

    View this table:
    Table 3

    Risk of adverse pregnancy outcomes among types of unemployment as compared to paid employment†

    In table 4 the adjusted risks of adverse pregnancy outcomes per type of unemployment as compared to housewives are presented. Job-seeking women had a higher risk of pre-eclampsia (OR 2.54, 95% CI 1.03 to 6.26) than housewives. Women receiving disability benefit were more likely to have preterm ruptured membranes (OR 3.16, 95% CI 1.49 to 6.70), elective caesarean section (OR 2.98, 95% CI 1.21 to 7.34) and preterm birth (OR 2.64, 95% CI 1.32 to 5.28) than housewives. The offspring of students and of women receiving disability benefit had a significantly lower mean birth weight than the offspring of housewives (difference: −93, 95% CI −174 to −12; and −97, 95% CI −190 to −5, respectively).

    View this table:
    Table 4

    Association between type of unemployment and pregnancy outcomes in the subgroup of unemployed pregnant women†

    Table 5 presents the adjusted associations between number of weekly working hours and pregnancy outcomes among the employed women. Of all employed participants, 26% (n=1136) reported working 1–24 h/week, 50% (n=2216) reported working 25–39 h/week and 26% (n=1056) reported working ≥40 h/week. Adjusted for confounders, weekly working hours remained significantly associated with birth weight: as the number of weekly working hours increased, mean birth weight decreased (p for trend=0.044). Compared with children born to women who worked 1–24 h/week during pregnancy, working ≥40 h/week was associated with a reduction in mean birth weight of 45 g (95% CI −89 to −1) in the offspring.

    View this table:
    Table 5

    Association between weekly working hours and pregnancy outcomes in subgroup of pregnant women with paid employment†

    Discussion

    The results of this large birth cohort study showed that, after correction for confounders, employed and unemployed women have the same risk of pregnancy complications, although within the subgroups of unemployed and employed women, we observed differences in pregnancy outcomes. Among unemployed pregnant women, those who receive disability benefit seem to be at highest risk of pregnancy complications. In the subgroup of women with paid employment, long weekly working hours during pregnancy were associated with a lower birth weight in the offspring.

    Employment status and pregnancy complications

    Regarding the association between employment status and pregnancy complications, a mixed pattern was observed. On the one hand, our study indicated that paid employment among pregnant women, as compared to unemployment, was associated with a lower risk of several pregnancy complications, that is, preterm ruptured membranes, preterm birth, meconium-stained amniotic fluid and SGA. Moreover, employment was associated with a higher mean birth weight than unemployment. These observations are along the lines of the ‘healthy worker effect’ that implies better health outcomes among employed persons.5 Nevertheless, caution is needed when interpreting the ‘healthy worker effect’, as it is frequently the consequence of confounding factors.23 24 Indeed, after adjustment for confounders, being employed was no longer associated with better pregnancy outcomes than being unemployed: the lower risks for pregnancy complications among employed women were entirely explained by epiphenomena of employment status, specifically the combination of more optimal socio-economic circumstances (ie, higher education and income) and the better mental health of these women.

    On the other hand, employed pregnant women also had an increased risk of adverse outcomes that mainly involved complications at delivery, such as poor progress of delivery and non-spontaneous delivery. These associations were entirely attributable to parity. The explanation for this is that the employed women in our study were more often nulliparous than unemployed women, and that giving birth for the first time is associated with a relatively high risk of such obstetric complications.25

    Defining statistical significance at a p value below 0.05, we found that employment status was not associated with pregnancy complications. However, some of the relationships just missed this level of statistical significance, that is the decreased risk of pregnancy-induced hypertension and non-optimal Apgar score as well as the increased risk of gestational diabetes and assisted vaginal delivery among employed women as compared to unemployed women. It might be that these associations did not reach statistical significance due to a small number of cases. Therefore, it is important that our findings are replicated in large population-based studies with a sufficient number of cases of these pregnancy complications.

    In the unemployed group, women receiving disability benefit had the highest risk of several pregnancy complications. The findings on mode of delivery indicate that obstetricians often decide in advance to end the pregnancies of women receiving disability benefit with an elective caesarean section. This may be done to reduce the health risks for both mother and child. Hypothetically, the ill health of women receiving disability benefit is the cause of both their unemployment and their increased risk of complicated pregnancies. Unfortunately, we were unable to control for the pre-existing physical health status of the pregnant women, as we lacked information on this topic. We also showed that the mean birth weight of the offspring of women receiving disability benefit and of students was lower as compared to the offspring of housewives. The explanation for this might be differences in levels of stress or variations in dietary and exercise patterns, which are known to be related to birth weight in offspring.3 26–28 Finally, the analyses among unemployed women indicated that offspring of students and of women receiving disability benefit had an increased risk of a non-optimal Apgar score after birth as compared to employed women. Further research is necessary to replicate this result, as this has, to our knowledge, not been reported before and because the analyses were performed on small groups.

    Among employed women, we observed that long weekly working hours during pregnancy were associated with a lower birth weight in the offspring. This is in contrast to earlier studies that reported no association.8 9 12 13 These studies, however, examined birth weight in a dichotomised way, while we analysed birth weight continuously whereby differences are detected more rapidly. Besides methodological considerations, such as multiple testing and residual confounding, there may be other reasons for the observed inverse association between working hours and birth weight. Women who work full-time might experience more stress than those working part-time. Stress during pregnancy is known to be associated with reduced blood flow through the uterine arteries.29 This may, in turn, affect fetal development and thus birth weight. Alternatively, the lower birth weight among women with long weekly working hours might also be explained by more frequent exposure to work-related hazards, such as prolonged standing12 or pesticides (A Burdorf et al, unpublished data 2009).

    Methodological considerations

    Several methodological considerations need to be addressed. The participants of the Generation R cohort represent a selection of a relatively healthy population.30 31 Moreover, the non-response analyses indicated that data were more complete in higher educated, non-single, older and non-smoking pregnant women of Dutch origin. Women with missing data on employment had a higher risk of some, but not all, pregnancy complications as compared to women without missing data. So, selective participation and response resulted in an under-representation of pregnant women of the most disadvantaged groups, who are at increased risk for pregnancy complications. This might restrict the external validity of our study, especially if the relationship between employment characteristics and pregnancy complications differed between participating and non-participating women. Despite our large study population, another limitation is the low prevalence rates of some of the pregnancy complications. For instance, only 66 women had gestational diabetes and only 63 babies had an Apgar score below 7 at 5 min after birth; consequently, there was limited power to detect significant differences. Moreover, while the diagnostic criteria used to identify cases of gestational diabetes in the present study compare well with those used by the American Diabetes Association,32 some cases of gestational diabetes may have been missed, as suggested by our relatively low incidence of gestational diabetes.33 This is probably because measurement of blood glucose levels was not a standard prenatal procedure, so that cases of gestational diabetes without overt symptoms might have remained unrecognised by the prenatal caregiver. Another shortcoming is that assessment of employment characteristics occurred at only one time point during pregnancy; however, we assume that these conditions generally remain relatively stable until maternity leave. Furthermore, the use of self-reports of alcohol consumption and smoking during pregnancy is also a limitation, as this may result in reporter bias. Finally, the adjustment for some confounding factors may have resulted in over-adjustment of the analyses. This pertains particularly to family income, as it is plausible that unemployment leads to pregnancy complications via restricted financial resources and worries about financial issues. Nevertheless, exclusion of family income from the adjusted analyses on the effect of employment status on pregnancy complications did not result in different findings and conclusions. The covariates educational level, parity and psychopathological symptoms may be, at least in part, preceding factors in the association between paid employment and pregnancy complications. However, as it is not probable that the effect of these covariates on pregnancy complications is completely mediated by employment status, these covariates should be considered as confounding factors rather than as antecedents.

    Implications and conclusions

    Because our study is one of the first to examine the relationship between employment status and weekly working hours with a wide range of pregnancy complications, more research is needed to replicate and complement our findings. It is important to elucidate the underlying mechanisms for our findings that women receiving disability benefit have an increased risk of pregnancy complications and that different types of unemployment and long weekly working hours are associated with a lower birth weight. Because commencement of maternity leave may be intertwined with the effect of weekly working hours on birth weight, future studies should also take this factor into consideration.

    In conclusion, we found no indications that paid employment during pregnancy benefits or endangers the health of the mother and child. The subgroups of employed and unemployed women proved, nevertheless, to be relatively heterogeneous: women receiving disability benefit had an elevated risk of preterm delivery compared to housewives. Moreover, the offspring of students, of women receiving disability benefit and of women with long working hours during pregnancy had a lower mean birth weight. More research is needed to confirm these findings and to provide explanations for these results. In the meantime, prenatal care providers should be made aware of the risk of pregnancy complications among women receiving disability benefit and students, and of the offspring's lower mean birth weight among women receiving disability benefit, students and women with long working hours during pregnancy. Perhaps extensive monitoring and counselling by prenatal care providers would lead to a reduction in the risks of pregnancy complications among these women. Future research should examine whether this is an effective strategy.

    What this paper adds

    • Research focussed on the relationship of employment status (employed versus unemployed) with gestational duration and birth weight has yielded inconsistent findings, while studies on the association between employment status and other pregnancy outcomes are lacking.

    • Our study showed that employed and unemployed women have the same risk of a wide range of pregnancy complications and adverse delivery outcomes.

    • The unemployed group proved to be relatively heterogeneous with women receiving disability benefit experiencing more complications than housewives; furthermore, offspring of both students and women receiving disability benefit had a significantly lower mean birth weight than the offspring of housewives.

    • Among women with paid employment, long working hours during pregnancy were associated with a lower mean birth weight in their offspring.

    Acknowledgments

    The Generation R Study is conducted by the Erasmus MC–University Medical Centre Rotterdam in close collaboration with: Erasmus University Rotterdam, School of Law and Faculty of Social Sciences; the Municipal Health Service Rotterdam area, Rotterdam; the Rotterdam Homecare Foundation, Rotterdam; and the Stichting Trombosedienst & Artsenlaboratorium Rijnmond (STAR), Rotterdam. We gratefully acknowledge the contribution of the participating pregnant women and their partners, general practitioners, hospitals, midwives and pharmacies in Rotterdam.

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    Footnotes

    • Funding The first phase of the Generation R Study is made possible by financial support from Erasmus MC–University Medical Centre Rotterdam, Erasmus University Rotterdam and the Netherlands Organization for Health Research and Development (ZonMW). The present study was supported by an additional grant from the Netherlands Organization for Health Research and Development (ZonMW “Geestkracht” program 10.000.1003).

    • Competing interests None.

    • Ethics approval This study was conducted with the approval of the Medical Ethical Committee of the Erasmus Medical Centre, Rotterdam.

    • Provenance and peer review Not commissioned; externally peer reviewed.