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Night work and sick leave during pregnancy: a national register-based within-worker cohort study
  1. Paula Edeusa Cristina Hammer1,
  2. Anne Helene Garde2,3,
  3. Luise Moelenberg Begtrup1,
  4. Esben Meulengracht Flachs1,
  5. Johnni Hansen4,
  6. Åse Marie Hansen2,3,
  7. Karin Sørig Hougaard2,
  8. Henrik A Kolstad5,
  9. Ann Dyreborg Larsen2,
  10. Anja Bisgaard Pinborg6,
  11. Ina Olmer Specht7,
  12. Jens Peter Bonde1,3
  1. 1 Department of Occupational and Environmental Medicine, Bispebjerg and Frederiksberg University Hospital, Copenhagen, Denmark
  2. 2 National Research Centre for the Working Environment, Copenhagen, Denmark
  3. 3 Department of Public Health, University of Copenhagen, Copenhagen, Denmark
  4. 4 Danish Cancer Society Research Centre, Copenhagen, Denmark
  5. 5 Department of Occupational Medicine, Danish Ramazzini Centre, Aarhus University Hospital, Aarhus, Denmark
  6. 6 Fertility Clinic, Rigshospitalet – Copenhagen University Hospital, Copenhagen, Denmark
  7. 7 The Parker Institute, Bispebjerg and Frederiksberg University Hospital, Copenhagen, Denmark
  1. Correspondence to Dr Paula Edeusa Cristina Hammer, Department of Occupational and Environmental Medicine, Bispebjerg and Frederiksberg University Hospital, 2400 Copenhagen, Denmark; paulahammer{at}


Objective The aim of our study was to investigate the acute effect of night work during pregnancy on the risk of calling in sick the following day using register-based information and the workers as their own controls.

Methods Using the payroll-based national Danish Working Hour Database, including all public hospital employees in Denmark, we identified 9799 pregnant women with ≥1 day shift and ≥1 night shift and ≥1 day of sick leave during the first 32 pregnancy weeks from January 2007 to December 2013. We performed fixed effects logistic regression, that is, within-worker comparisons, of the risk of sick leave of any duration starting within 24 hours after night shifts of different length versus day shifts.

Results Most of the participants were nurses (64%) or physicians (16%). We found an increased relative risk of sick leave following night shifts compared with day shifts during all pregnancy trimesters. The risk was highest for night shifts lasting >12 hours (OR 1.37, 95% CI 1.15 to 1.63 for nurses; OR 1.87, 95% CI 1.69 to 2.08 for physicians) and among women aged >35 years (OR 1.42, 95% CI 1.24 to 1.63).

Conclusion Among Danish public hospital employees night shifts during pregnancy, especially shifts longer than 12 hours, increased the risk of calling in sick the following day independent of personal factors and time-invariant confounders in all pregnancy trimesters.

  • shift work
  • female reproductive effects and adverse pregnancy outcomes
  • sickness absence
  • sick leave
  • night work

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  • Contributors PECH, JPB, AHG, EMF, ADL and LMB provided substantial contributions to the conception of the study and the analysis of the data. PECH, AHG and ADL contributed to the acquisition of the data. PECH was responsible for drafting the paper. All authors contributed substantially to the interpretation of the data, the critical revision of the paper for important intellectual content and the final approval of the version published. All authors are accountable for all aspects of the work.

  • Funding This work was financially supported by The Danish Working Environment Research Fund grant 31-2015-03 2015001705. The establishment of the DWHD has been financed by research grants from The Danish Working Environment Research Fund (grant 23-2012-09), The Nordic Program on Health and Welfare – Nordforsk (grant 74809) and The National Research Centre for the Working Environment. The Danish administrative regions have partially financed the transfer of data to the cohort.

  • Competing interests None declared.

  • Patient consent Not required.

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