Objective Observational 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.
Methods A 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.
Results In 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.
Conclusions The 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.
- night work
- payroll data
- cohort study
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Contributors LMB, JPB, PECH, EMF and IOS conceived and designed the study. AHG, JH, ÅMH, HAK established and provided data from the DWHD. JPB analysed the data and EMF gave statistical support. LMB drafted the manuscript and all authors interpreted the data and revised the manuscript.
Funding This work was supported by the Danish Working Environment Research Fund grant 31-2015-03 2015001705.
Competing interests None declared.
Ethics approval The study was approved by the Danish Data Protection Agency (though the notification system in the Capital region of Denmark, j.nr.: 2012-58-0004). By Danish law, no informed consent is required for a register-based study.
Provenance and peer review Not commissioned; externally peer reviewed.
Presented at The results were presented at the Reproyoung conference, Ystad 25th – 26th October.
Patient consent for publication Not required.