In case expectant mothers didn’t have enough to worry about, researchers at the University of Auckland have found a link between sleep position in the final hours of pregnancy and the risk of late stillbirth.

Women who did not sleep on their left side on their last night of pregnancy had twice the risk of late stillbirth compared with women who did sleep on their left side, the study, published online by BMJ, found.

The overall risk was very small, however: there were 1.96 stillbirths per 1,000 women who slept on their left side just before giving birth, compared with 3.93 stillbirths per 1,000 women who slept in any other position.

Still, the findings were a surprise to the scientific community. “This is a new observation and given that stillbirth rates in New Zealand have not changed in 20 years, it is a dramatic result,” said child health researcher and study co-author Ed Mitchell of Cure Kids, which funded the study, in a statement.

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The researchers interviewed 155 women in Auckland who experienced stillbirth at 28 weeks or later, between July 2006 and June 2009; 310 women with ongoing pregnancies were also interviewed as controls. The women were asked about their sleep positions (left side, right side, back or other), and about their nighttime sleep and wake habits during the last month, week and night before they believed their baby had died.

The team also included questions about snoring, daytime sleepiness and duration of sleep, and frequency of nighttime visits to the bathroom. The researchers found no association between snoring or daytime sleepiness and stillbirth risk. But they did find that women who went to the toilet once or less on the last night of pregnancy were more likely to have a stillborn baby, compared with women who went to the bathroom more frequently. The study also found a higher risk of stillbirth in women who regularly slept during the day and slept for longer than average at night.

As for the increased risk associated with sleep position, the authors suggested that it may have to do with restricted blood flow to the fetus when the mother lies on her back or right side for extended periods.

But the authors caution that pregnant women shouldn’t become anxious about sleeping. The new study is the first of its kind, and is only a starting point for future research. “It was an observational study, not one that can show cause and effect — all it does is show an association. It would be premature to jump up and down and say that everyone has got to sleep on their left,” lead author Tomasina Stacey of the University of Auckland told the Guardian newspaper.

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It’s worth noting also that the women’s sleep information was collected by self-report. As one U.K. physician commented online in response to the paper, “[W]e have no idea how many women can accurately recollect how they fell asleep (can any of us?) the night before their baby died, as long after the event as three weeks. Many of us cannot recall what we had for dinner two nights ago.”

Stacey’s team is now seeking funding to conduct larger, more detailed studies. The World Health Organization found in a series of studies earlier this year that more than 2.6 million pregnancies end in stillbirth each year, mostly among women in poor countries. That adds up to more than 7,200 stillborn babies every day, so concrete findings about reducing that risk are necessary.

In an accompanying commentary on BMJ.com, Lucy Chappell, a clinical senior lecturer in maternal and fetal medicine at King’s College London, noted that “any simple intervention that reduces the risk of stillbirth would be extremely welcome,” but that “a forceful campaign urging pregnant women to sleep on their left side is not yet warranted.”

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