(HealthDay)—Women who sleep on their backs in the later months of pregnancy may have a relatively higher risk of stillbirth if they already have other risk factors, a new study suggests.
Experts stressed that the findings do not prove that sleep position itself affects stillbirth risk.
"We should be cautious in interpreting the results," said Dr. George Saade, director of maternal-fetal medicine at the University of Texas Medical Branch at Galveston.
"We can't conclude that sleeping on the back causes stillbirth, or that sleeping on your side will prevent it," said Saade, who was not involved in the study.
It is, however, plausible that back-sleeping could contribute, Saade said. Lying on the back can exacerbate sleep apnea, where breathing repeatedly stops and starts throughout the night, and if a fetus is already vulnerable, that reduced oxygen flow could conceivably boost the odds of stillbirth, he explained.
Dr. Adrienne Gordon, the lead researcher on the study, agreed that if sleep position contributes to stillbirth, it would probably be only if other risk factors are present, such as impaired growth of the fetus.
"Stillbirth is much more complicated than one risk factor," said Gordon, a neonatologist at Royal Prince Alfred Hospital in Sydney, Australia.
But if sleep position does matter, she added, that would be important because it can be changed.
Stillbirth refers to a pregnancy loss after the 20th week. According to the March of Dimes, about one in 160 pregnancies ends in stillbirth—with birth defects, poor fetal growth and problems with the placenta among the causes.
Women who smoke or have high blood pressure are at greater risk than others, but sometimes there is no explanation for a stillbirth, Saade said.
To see whether sleep position is connected to stillbirth risk, Gordon's team studied 103 women who had suffered a late stillbirth—after the 31st week of pregnancy—and 192 pregnant women who were in the third trimester.
They found that of women who had a stillbirth, almost 10 percent said they had slept on their backs during pregnancy, including the last month. That compared with only 2 percent of women with healthy pregnancies.
When the researchers accounted for other factors—such as smoking and women's body weight—back-sleeping was still linked to an increased risk of stillbirth.
Dr. Halit Pinar, director of perinatal and pediatric pathology at Women and Infants Hospital in Providence, R.I., studies potential risk factors for stillbirth.
He said his research has found that impaired fetal growth is a "major risk factor" for stillbirth—a link that Gordon's team saw in the current study as well.
When it comes to sleep position, Pinar said the current findings raise an interesting question, but that's as far as they go.
According to Pinar, it's "feasible" that blood flow to the fetus could be diminished when a woman sleeps on her back.
"But without any objective evidence, such as measuring the actual flow to the placenta and the baby, it's hard to accept that without some trepidation," Pinar noted.
"At this stage," he said, "I don't think we can reach any conclusions about the effect of sleep position and come up with a recommendation."
Gordon and Saade agreed that it's too early for any sweeping recommendations.
"I don't think women should be alarmed" by the findings, Saade said. "And a woman who has had a stillbirth should definitely not feel guilty if she slept on her back during pregnancy."
But should women sleep on their side, just to be safe? Not necessarily, Saade said. That sleep position could potentially encourage a blood clot in the legs, he noted.
"Women should sleep in whatever position is comfortable for them," Saade said.
However, if a woman has any concerns about her sleep position, experts say she should discuss it with her doctor.
The study was published Jan. 8 online in Obstetrics & Gynecology.
Explore further: Ghanaian pregnant women who sleep on back at increased risk of stillbirth
The March of Dimes has more on stillbirth.