According to Science Daily, the small-scale study, published in the Journal of Physiology, is the first to monitor unborn babies overnight and at the same time, record the mother’s position during sleep.
Lead author Professor Peter Stone said: “We are suggesting that there is now sufficient evidence to recommend mothers avoid sleeping on their back in late pregnancy.”
The researchers analysed 29 women who were between 34 and 38 weeks pregnant. Their babies’ heart rates were continuously recorded overnight and their sleeping positions were monitored via video recordings.
Researchers found when a woman slept on her back, their baby was less active than when they slept on their side.
If a mum changed position during sleep - from being on her side to going onto her back - the baby became “quieter”.
Professor Stone said: “In the situation where the baby may not be healthy, such as those with poor growth, the baby may not tolerate the effect of maternal back-sleeping.”
Louise Silverton, director for Midwifery at the Royal College of Midwives (RCM) welcomed the research, despite it being a small study.
“The potential for an increased risk of stillbirth as a result of a pregnant woman’s sleeping position is an important one and the RCM welcomes this research that supports earlier studies,” she told HuffPost UK.
“We have known for a long time that blood pressure is reduced where a woman is lying on her back.
“Many women will say that they don’t lie on their backs as they don’t always feel most comfortable in that sleeping position and often women are advised not to sleep on their backs.”
“This small study adds to information in this area. Women should try to lie on their side when going to sleep.
“They will change position unconsciously whilst asleep and we believe that most will avoid moving onto their backs whilst asleep.
“Also, it is known that when in labour, moving a woman to the left hand side may improve the foetal heart rate trace.
“What this study shows is that even in the healthiest of women with a foetus with no known risk factors, position is important.
“Equally, speaking to your midwife and discussing the best sleeping and resting positions for even something as simple lying on a sofa during pregnancy is crucial.
“The RCM would like to see more research focusing on sleeping position during late pregnancy and the effects that it may have on both mother and baby. Anyone women with concerns should speak to their midwife or GP.”