Expectant mothers should sleep on their side towards the end of pregnancy, experts have said after a new study highlighted the dangers of lying flat on their back.
Researchers monitored both unborn babies and their mothers overnight to observe the effects of sleeping positions.
The study, published in The Journal of Physiology, found that women who sleep on their backs in late pregnancy may cause problems for the baby.
Previous studies have linked the position a mother sleeps in with an impact on the baby's risk of still birth - compared with sleeping on the left side, lying flat facing upwards was found to be linked to an increased risk of stillbirth.
In the new study, researchers decided to monitor the immediate impact sleeping positions have on a baby by measuring them overnight.
The research saw 29 healthy women who were 34 to 38 weeks pregnant given fetal electrocardiogram recordings overnight to monitor the heart activity.
Researchers from the University of Auckland also observed maternal sleeping positions using an infrared camera.
They found that when the mother slept on her back, the foetus was less active.
Foetuses were only in an active state when the mother was on her left or right side.
When the mother changed position during sleep, for example from her left side to sleeping on her back, the baby quickly changed activity state and became quiet or still, they found.
"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," said Peter Stone, one of the study's lead investigators.
"We are suggesting that there is now sufficient evidence to recommend mothers avoid sleeping on their back in late pregnancy, not only because of the epidemiological data but also because we have shown it has a clear effect on the baby."
