Heavily pregnant women should not sleep on their back as this could increase the risk of late stillbirth, new research warned.

Instead women in the third trimester should sleep on their side to protect their unborn child.

Scientists found unborn babies in the late stages of pregnancy are less active in the womb if their mother’s lie on their backs – a measure of their wellbeing.

The study found the babies 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 – a “supine position” – the baby quickly changed becoming quiet or still.

Professor Peter Stone at the University of Auckland said: “Maternal sleep position in late pregnancy is now recognised as an important risk factor for stillbirth. Compared with commencing sleep on the left side, the supine sleep position was found to have an increased risk of stillbirth.

“The mechanisms by which maternal position could influence foetal outcome were unclear.

“Whilst the reasons by which maternal position may affect foetal outcome remain unclear, the supine position may reduce maternal cardiac output and uteroplacental perfusion and result in a hypoxicstress for the fetus.”

Mr Stone investigated the sleep position of 30 pregnant women after 28 weeks gestation by setting up an infrared video camera to record their position as they slept. They also recorded the heart rate of both the expectant mother and their unborn child overnight using an ECG device.

The women were followed at 34 to 38 weeks gestation, and all of them were healthy with healthy babies.

The study also found that vulnerable foetuses which are already suffering from low oxygen levels could be unable to adapt to the stress of the mother sleeping on her back.

The researchers are now investigating pregnancies where the foetus is not growing properly or the mother has reported decreased foetal movements - both of which are associated with an increased risk of stillbirth.

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.

“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.

“The findings in our study of healthy late gestation pregnancy are consistent with the concept that state changes can occur as an adaptive response and shift the foetus to a lower oxygen consuming state.

“Decreased fetal movements are associated with adverse pregnancy outcomes including hypoxia at birth or stillbirth.

“It has been further suggested that placentae from pregnancies affected by reduced foetal movements are associated with abnormal placental structure and impaired placental function.

“Given our results, we suggest that trials of interventions to reduce supine sleeping in the third trimester of pregnancy are warranted.

“Our findings suggest that a supine sleep position is disadvantageous even to the healthy late gestation fetus, but whether left or right lateral positions have significantly different effects on fetal behaviour is unclear.”

The study was published in The Journal of Physiology.