Women who lie on their backs in the last three months of pregnancy may have a higher risk of stillbirth, research suggests.
A study by NZ researchers found that lying on your 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."
"Further research is needed to see the effect of staying in certain maternal sleeping positions overnight."