Working as little as even two night shifts a week significantly increases a woman’s risk of miscarriage, Danish researchers say.

Based on their analysis of data from a cohort of over 22,000 pregnant women primarily employed at hospitals, researchers found those women who worked two or more night shifts the previous week were 32% more like to have a miscarriage after week eight of their pregnancy, compared to women who did not work night shifts.

In addition, increasing the number of night shifts and the number of consecutive night shifts during weeks three to 21 increased the risk of miscarriage even further in a dose-dependent manner.

Interestingly, the study found no association between miscarriage and quick returns after a night shift (eg doing an evening shift after having completed a night shift the night before).

The study findings, published recently in BMJ publication, Occupational and Environmental Medicine, support previous research that suggested a link between working nights and miscarriage. But previous studies had been limited by a lack of detailed data about the women’s exposure to night work which had meant the link between cause and effect could not be confirmed.

Miscarriages are known to be very common with the researchers quoting the estimate that about one third of all human embryos are lost, most of them soon after conception. They also quote the figure that more than half of all miscarriages are the result of chromosomal abnormalities in the fetus.

The finding of the association between night work and miscarriage at week eight supports the hypothesis that it is the environmental exposure that is the risk factor, as miscarriages associated with chromosomal abnormalities have generally occurred earlier in the pregnancy.

Night work is believed to be a threat to the viability of pregnancies because of its effect on maternal levels of endogenous melatonin, a hormone thought to play a role in optimal function of the placenta. Exposure to light at night along with the disruption of the normal circadian sleep-wake cycles associated with night work both decrease melatonin release.

The study potentially has significant implications and ramifications.

“This new knowledge has relevance for working pregnant women as well as their employers, physicians and midwifes,” the study authors said.

“Moreover, the results could have implications for national occupational health regulations,” they concluded.

Reference

Begtrup LM, Specht IO, Hammer PEC, Flachs EM, Garde AH, Hansen J, et al. Night work and miscarriage: a Danish nationwide register-based cohort study. Occup Environ Med. 2019 Mar 29. DOI: 10.1136/oemed-2018-105592 [epub ahead of print]