Pregnant women who regularly work the night shift may have an increased risk of a miscarriage late in pregnancy or a stillbirth, a new study suggests.

The study of more than 40,000 Danish women who worked during pregnancy found that those who consistently worked the graveyard shift were 85 percent more likely than daytime workers to suffer a miscarriage relatively late in pregnancy or have a stillbirth.

Other job shifts - including rotating shifts that required some overnight work - were not related to late pregnancy loss, according to findings published in the Journal of Occupational and Environmental Medicine.

Overall, just over 1 percent of the pregnancies ended in miscarriage or stillbirth, and only 11 of the 420 women who worked a fixed night shift suffered a pregnancy loss.

Still, when the researchers weighed other factors such as the woman’s age, smoking habits and the physical demands of the job, overnight work was linked to an 85 percent higher risk of pregnancy loss compared with fixed daytime work.

“The results from our study are in favor of an effect (of) night work,” the study’s lead author, Jin Liang Zhu of the University of Aarhus, told Reuters Health.

The findings are in line with a number of studies suggesting that working the late shift, or rotating shifts that include night work, can take a toll on the body, including increasing the risk of digestive problems, heart disease and certain cancers. In a recent analysis of the same group of Danish women, Zhu’s team found that those who worked nights or rotating shifts had a slightly higher risk of having a low-birthweight baby.

It’s thought that nighttime work may promote health problems by throwing off the body’s circadian rhythms - daily physiological patterns, governed by the body’s internal “clock,” that not only help control the sleep/wake cycle, but also influence a range of body processes, including blood pressure changes and hormone production.

According to Zhu’s team, the link between pregnancy loss and steady overnight work may have to do with estrogen levels. Exposure to light at night suppresses the normal nighttime release of the sleep-related hormone melatonin, which in turn is believed to spur an increase in other hormones, including estrogen.

Some recent research has found a heightened risk of breast cancer among women who work mainly at night. Effects on estrogen levels have been proposed as one explanation for that finding.

As for why women on rotating shifts in this study did not have a higher risk of pregnancy loss, Zhu said it’s possible that a rotating schedule may fall short of any “threshold” at which night work begins to have an effect.

The researchers also found evidence that job stress could be a factor in night-shift workers’ higher risk of miscarriage and stillbirth. Job stress was not linked to pregnancy loss overall, but among workers on fixed night shifts, those who said their jobs had high demands but gave them little control over their work had a higher risk of pregnancy loss.

“We believe that job stress involved in working on a fixed night schedule may be larger than the job stress reported by all in general,” Zhu said.

However, the researchers say the finding on job strain “must be interpreted cautiously” because of the small number of women the sub-analysis involved.

SOURCE: Journal of Occupational and Environmental Medicine, November 2004.