Night work and miscarriage: a Danish nationwide register-based cohort study

Abstract

ObjectiveObservational studies indicate an association between working nights and miscarriage, but inaccurate exposure assessment precludes causal inference. Using payroll data with exact and prospective measurement of night work, the objective was to investigate whether working night shifts during pregnancy increases the risk of miscarriage.MethodsA cohort of 22 744 pregnant women was identified by linking the Danish Working Hour Database (DWHD), which holds payroll data on all Danish public hospital employees, with Danish national registers on births and admissions to hospitals (miscarriage). The risk of miscarriage during pregnancy weeks 4–22 according to measures of night work was analysed using Cox regression with time-varying exposure adjusted for a fixed set of potential confounders.ResultsIn total 377 896 pregnancy weeks (average 19.7) were available for follow-up. Women who had two or more night shifts the previous week had an increased risk of miscarriage after pregnancy week 8 (HR 1.32 (95% CI 1.07 to 1.62) compared with women, who did not work night shifts. The cumulated number of night shifts during pregnancy weeks 3–21 increased the risk of miscarriages in a dose-dependent pattern.ConclusionsThe study corroborates earlier findings that night work during pregnancy may confer an increased risk of miscarriage and indicates a lowest observed threshold level of two night shifts per week.

Effects of long working hours and shift work during pregnancy on obstetric and perinatal outcomes: A large prospective cohort study—Japan Environment and Children’s Study

“…A recent report suggested that night work was not associated with an elevated risk of preterm birth. 11 In contrast, Begtrup et al 12 suggested that night workers might have an increased risk of miscarriage compared with the risk in pregnant women not working night shifts. The result in the present study was concordant with that of the former report despite the increased risks of threatened miscarriage and threatened preterm labor, which might be a result of obstetricians prescribing complete rest for most pregnant women facing threatened miscarriage or threatened preterm labor with signs of genital bleeding.…”

“…In addition, the majority of pregnant women were recruited after the latter half of the first trimester, which means that early miscarriages were not covered in the JECS. Thus, we did not analyze the risk of early miscarriages despite its importance as an occupational health issue …”

“…With respect to shift work in women, shift workers may have increased rates of menstrual disruption and night shifts may be associated with increased rates of spontaneous abortion . However, whether these results are applicable to other settings remains controversial as another study showed no significant increase in miscarriage rates among shift work nurses .…”

“…2 Furthermore, evidence has been accumulating from systematic reviews, meta-analyses, and register-based prospective cohort studies about obstetric outcomes of pregnant workers. [9][10][11][12][13] With respect to shift work in women, shift workers may have increased rates of menstrual disruption and night shifts may be associated with increased rates of spontaneous abortion. 9,10,12,13 However, whether these results are applicable to other settings remains controversial as another study showed no significant increase in miscarriage rates among shift work nurses.…”

“…[9][10][11][12][13] With respect to shift work in women, shift workers may have increased rates of menstrual disruption and night shifts may be associated with increased rates of spontaneous abortion. 9,10,12,13 However, whether these results are applicable to other settings remains controversial as another study showed no significant increase in miscarriage rates among shift work nurses. 14 Because research on the relationship between shift work and pregnancy outcomes is limited, the present study was conducted to provide a scientific basis for better worklife balance in pregnant women and key precautions for prenatal care.…”