Pregnant women who work night shifts may have an increased risk for miscarriage.

Researchers studied 22,744 pregnant Danish women, tracking their work schedules and hospital admissions for miscarriage using government databases. The study, in Occupational & Environmental Medicine, found that after eight weeks of pregnancy, women who had worked two or more night shifts during the previous week had a 32 per cent increased risk of miscarriage compared with women who did not work nights. Working only one night shift a week did not significantly increase the risk.

The study adjusted for maternal age, previous miscarriages, smoking, socioeconomic status, body mass index and other factors.

The lead author, Luise Moelenberg Begtrup, a postdoctoral researcher at Bispebjerg Hospital in Copenhagen, said that this is an observational study that does not prove causality, and that it has to be replicated in other studies and in other populations. Most of the women in this study were health care workers. Why this happens is not known, but irregular sleep may affect production of the hormone melatonin, which is thought to play a role in maintaining optimal function of the placenta.

“We are not recommending that you should avoid night shifts entirely,” Begtrup said. “But organising them properly is important. Our data was very strong and my recommendation at this time would be only one night shift a week.”