Women who lie on their backs in the last three months of pregnancy may have a higher risk of stillbirth, research suggests.
The study found that lying on the back leads unborn babies to adopt a more dormant or inactive state, possibly caused by receiving lower levels of oxygen.
These lower oxygen levels cause the foetus to move "to a low oxygen consuming state", while the baby's heart rate is also affected, according to the study, published in the Journal of Physiology.
Researchers at the University of Auckland analysed data for 29 healthy pregnant women in their third trimester. The women's heart rate and their baby's heart rate were monitored while they rested in four positions, changing them every 30 minutes.
The positions were lying on the left side, lying on the right, lying on the back but propped up at a 30 degree angle with a pillow, and lying flat with a pillow.
The results showed that "maternal position has a significant relationship with both foetal behavioural state as determined by features of foetal heart rate and its variability".
Peter Stone, professor of maternal foetal medicine at the University of Auckland and lead investigator of the study, said: "Our controlled study found that lying on your back can add extra stress to the baby, contributing to the risk of stillbirth. The risk is likely to be increased further in women with underlying conditions.
"We have only looked at the effect of maternal positions for a short period of time while the mother is awake. Further research is needed to see the effect of staying in certain maternal sleeping positions overnight."
Louise Silverton, director for midwifery at the Royal College of Midwives (RCM), said: "This is an interesting study that provides some clear information regarding maternal position and its effects of the foetus in late pregnancy.
"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 feel 'funny' when they do.
"Women are advised not to sleep on their backs. Also, it is known that, when in labour, moving a woman to her left-hand side may improve the foetal heart rate trace.
"What this study shows that is different is that, even in the healthiest of women with a foetus with no known risk factors, position is important.
"There is a lesson here for midwives in that we must ensure that we advise women on the best positions for sleep as well as for lying on a sofa."
Hannah Knight, from the Royal College of Obstetricians and Gynaecologists (RCOG), said: "This is not the first piece of research to suggest a possible link between a mother's sleeping position and stillbirth risk.
"This study suggests that in late pregnancy, the heart rate of a foetus can change when a mother's position changes.
"It is important that women are not unnecessarily alarmed by the results of this small study of 29 women, none of whom had a stillbirth.
"Instead, it measures the incidence of babies in a sleep-like state, which has little evidence of any association with hypoxia, brain damage or stillbirth.
"It is not possible to draw a firm link between maternal position and stillbirth risk from this study and further robust research is needed."