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Original research
Night work during pregnancy and small for gestational age: a Danish nationwide register-based cohort study
  1. Luise Moelenberg Begtrup1,2,
  2. Camilla Sandal Sejbaek1,
  3. Esben Meulengracht Flachs1,
  4. Anne Helene Garde2,3,
  5. Ina Olmer Specht4,5,
  6. Johnni Hansen6,
  7. Henrik A Kolstad7,8,
  8. Jens Peter Ellekilde Bonde1,2,
  9. Paula Edeusa Cristina Hammer1
  1. 1 Department of Occupational and Environmental Medicine, Copenhagen University Hospital Bispebjerg, Copenhagen, Denmark
  2. 2 Department of Public Health, Copenhagen University, Copenhagen, Denmark
  3. 3 National Research Centre for the Working Environment, Kobenhavn, Denmark
  4. 4 The Parker Institute, Frederiksberg Hospital, Frederiksberg, Denmark
  5. 5 Section for General Practice, Department of Public Health, Copenhagen University, Copenhagen, Denmark
  6. 6 Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen, Denmark
  7. 7 Department of Occupational Medicine, Danish Ramazzini Centre, Aarhus University Hospital, Aarhus, Denmark
  8. 8 Insitute of Clinical Medicine, Aarhus University, Aarhus, Denmark
  1. Correspondence to Dr Luise Moelenberg Begtrup, Bispebjerg and Frederiksberg, Department of Occupational and Evironmental Medicine, Copenhagen University Hospital, Kobenhavn 2400, Denmark; luise.moelenberg.begtrup.02{at}


Objective The aim was to investigate the association between night work during pregnancy and risk of having a small for gestational age (SGA) child.

Methods This cohort study had payroll data with detailed information on working hours for employees in all Danish administrative regions (primarily hospital employees) between 2007 and 2015, retrieved from the Danish Working Hour Database. Pregnancies, covariates and outcome were identified from the national birth registry. We used logistic regression to investigate the association between intensity and duration of night work during the first 32 pregnancy weeks and SGA. The adjusted model included age, body mass index, socioeconomic status and smoking. Using quantitative bias analysis and G-estimation, we explored potential healthy worker survivor bias (HWSB).

Results The final cohort comprised 24 548 singleton pregnancies in 19 107 women, primarily nurses and medical doctors. None of the dimensions of night work were associated with an increased risk of SGA. We found a tendency towards higher risk of SGA in pregnancies where the women stopped having night shifts during pregnancy. Using G-estimation we found an OR<1 for the association between night work and SGA if all workers continued having night work during pregnancy compared with daywork only.

Conclusion We found no increased risk of SGA in association with night work during pregnancy among healthcare workers. G-estimation was not precise enough to estimate the observed indication of HWSB. We need better data on pregnancy discomforts and complications to be able to safely rule out HWSB.

  • Occupational Health
  • Shift Work Schedule
  • Pregnancy Outcome
  • Reproductive Medicine
  • Obstetrics

Data availability statement

Data may be obtained from a third party and are not publicly available.

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Data availability statement

Data may be obtained from a third party and are not publicly available.

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  • Contributors LMB, JPEB, PECH, CSS and EMF conceived and designed the study. AHG, JH and HAK established and provided data from the DWHD. PECH, LMB and EMF analysed the data and EMF gave statistical support. LMB drafted the manuscript, and all authors interpreted the data and revised and accepted the final the manuscript. LMB is responsible for the overall content as the guarantor.

  • Funding This work was supported by the Danish Working Environment Research Fund grant 31-2015-03 2015001705.

  • Competing interests None declared.

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

  • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.