Women who slept on their right side or on their back were no more likely to experience serious complications than women who slept on their left side, the study found.

These results should reassure many pregnant women who might worry about harming their baby by sleeping on their back, or moving into this position during the night, said Robert Silver, lead author of the study and chair of obstetrics and gynecology at the University of Utah School of Medicine in Salt Lake City.

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“There is downside to encouraging the avoidance of supine [back] sleep,” Silver said by email.

“Some women may have trouble sleeping on their left side and they cannot control movement during sleep,” Silver said. “Even with careful messaging, there is potential to increase anxiety in women who wake up on their backs and guilt, shame and self-blame in women suffering adverse pregnancy outcomes such as stillbirth.”

Previous studies have linked sleeping on the back or right side to an increased risk of serious pregnancy complications because these positions may compress blood vessels supplying the uterus, researchers said in Obstetrics and Gynecology.

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Women in the current study were more likely to have serious pregnancy complications when they were overweight or obese, smoked or had high blood pressure or diabetes before pregnancy.

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Neither the positions women were in when they went to sleep or woke up, nor the positions they might move to during the night appeared to impact the risk of complications.

Researchers also looked at objectively measured sleep positions for a subset of women who underwent home sleep studies for nighttime breathing problems. For these women, there also was no meaningful difference in the risk of pregnancy complications based on whether they slept on their back more than half of the time, or less often.

The study wasn’t designed to prove whether or how sleep positions might directly impact pregnancy outcomes. It also didn’t look at the connection between sleep position and complications in the final weeks of pregnancy.