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Researchers do say that pregnant women shouldn’t be too alarmed by the findings. The chances of miscarriage are still relatively small. Getty Images

Pregnant women who work at least two night shifts a week may be at an increased risk of miscarriage, although the chances of a miscarriage are still relatively small.

Researchers in Denmark came to that conclusion after studying data of almost 23,000 pregnant women to see how evening work impacted the chance of a miscarriage between the 4th and 22nd weeks of pregnancy.

The researchers found that among women more than eight weeks pregnant, those who worked two or more night shifts in the past week had a 32 percent greater risk of miscarriage than pregnant women who didn’t work the so-called swing shift.

The risk increased with the number of night shifts worked each week, as well as the number of consecutive night shifts worked.

“Women working night shifts are exposed to light at night, which disrupts their circadian rhythm and decreases the release of melatonin. Melatonin has been shown to be important in maintaining a successful pregnancy, possibly by preserving the function of the placenta,” Dr. Luise Molenberg Begtrup, study author and researcher at the Department of Occupational and Environmental Medicine at Bispebjerg and Frederiksberg Hospital in Denmark, told Healthline.

However, the researchers said pregnant women shouldn’t be alarmed by their findings.

“Working night shifts seems to carry an increased risk of spontaneous abortion — but the magnitude is not alarming. Among 100 women with night shifts we would expect one extra case than among women only working day shifts, namely in the range of five rather than four spontaneous abortions after gestational week eight,” Molenberg Begtrup said.

According to the U.S. Centers for Disease Control and Prevention (CDC), working at night or working long hours has been related to preterm birth, menstrual disorders, and miscarriages.

Working at night can disrupt the body’s circadian rhythm, which helps regulate hormones in pregnancy.

Dr. Shannon Clark, an associate professor in the department of obstetrics and gynecology at University of Texas Medical Branch at Galveston, said the amount of sleep needed during pregnancy varies.

“Women do require more sleep during pregnancy, particularly in the first and third trimester,” she told Healthline. “Fatigue is more evident in these trimesters when compared to the second trimester, when many women experience more energy. As to exactly how many hours a day is needed, it varies from woman to woman based on her baseline needs when not pregnant.”

In the Danish study, of the 10,047 women who worked some night shifts between the 3rd and 21st weeks of pregnancy, there were 740 miscarriages.

The study also found women who worked 26 or more night shifts between week 4 and week 22 of pregnancy were twice as likely to miscarry when compared with those who didn’t work nights, though this was only based on the experience of eight women studied.

Although the research shows an association between night shifts and miscarriage, this isn’t proof of causation and experts have cautioned that pregnant women shouldn’t be alarmed if they work night shifts.

“I am currently not convinced that there is any significant harm to pregnant mothers who work night shifts. I base this opinion on my personal clinical experiences and on my review of past and current literature that has focused on pregnancy outcomes in mothers who work night shifts.” Dr. Marc Parrish, an associate professor of obstetrics and gynecology at the University of Kansas Medical Center, told Healthline.

“What I have taken away from the literature is that there is a lot of inconsistency in what is being reported, with some studies suggesting harm, and others not,” he said.

About 30 percent of workers in the United States get less than six hours of sleep a night. About 15 million work full-time on night shifts, irregular schedules, or rotating shifts.

Dr. Sheri Belafsky, director of the Medical Surveillance Program in the Center for Occupational and Environmental Health, Department of Public Health Sciences at the University of California Davis, says further research following the Danish study would be beneficial.

“I was definitely surprised by these results and hope follow-up studies will help pinpoint what exactly it is about night shift work that may increase the risk of miscarriage. This study population was comprised of nurses and physicians, and I’m curious about the type of work they were specifically doing at night,” she told Healthline.

The Danish research is the latest in a line of many studies examining night shift work in women of reproductive age.

None of these have been randomized controlled trials and as such cannot be definitively conclusive.

Molenberg Begtrup notes it’s unethical to do randomized intervention studies that examine negative impacts of exposures on pregnancies.

For some women, working night shifts while pregnant may be the only option. Clark, who’s also a spokesperson for the American College of Obstetricians and Gynecologists, says there are a number of ways these women can ensure they remain healthy during pregnancy.

“I would advise her to maintain a regular sleeping pattern as much as possible, refrain from alternating day shifts and night shifts, maintain a healthy weight, have routine obstetrical visits, and make sure that any pre-existing medical conditions, like diabetes or hypertension, are optimally controlled prior to pregnancy,” she said.

“Overall, working a fixed night shift schedule rather than an alternating night shift schedule may be less stressful on the body and may be preferred, especially during pregnancy,” she added.