Objective Few studies investigated hypertensive disorders of pregnancy (HDP) in relation to work at night with inconclusive results and crude exposure assessment. Our aim is to investigate the risk of HDP after work at night during pregnancy based on objective exposure assessment from The Danish Working Hour Database (DWHD), which contains information on working hours from all public hospital employees in Denmark.
Methods The study population (n=20,385) comprised women from DWHD who have given birth at least once between 2007 and 2013. Night and day shifts were defined as at least three hours between 00:00 and 05:00 and between 06:00 and 20:00 respectively. Cases of HDP defined as gestational hypertension or preeclampsia/eclampsia were retrieved from The Danish National Patient Registry. We analysed the risk of HDP by number of night shifts during the first 20 weeks of gestation by logistic regression adjusted for relevant covariates.
Results The risk of HDP among women working 1–3 and>=4 night shifts during the first 20 pregnancy weeks was OR=0.94 (95%CI 0.77, 1.16) and OR=1.03 (0.75, 1.41), respectively, compared to day workers. Stratified analyses revealed an increased risk of HDP among women older than 35 years who worked at night compared to day workers (OR=1.76; 1.05, 3.04 p value for interaction <0.001).
Conclusion Our results of no overall increased risk of HDP among night workers are reassuring. The post hoc result finding of increased risk among women older than 35 years needs cautious interpretation.
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