Women who go to sleep on their back in the final three months of pregnancy are almost four times more likely to have a stillbirth, new research has found.
The study also debunks earlier research suggesting sleeping on the left side was better for babies than the right.
The New Zealand Multi-Centre Stillbirth Study, led by Auckland University's head of obstetrics and gynaecology Professor Lesley McCowan, found that going to sleep lying on your back was associated with a 3.7-fold increase in the risk of a stillbirth after 28 weeks of pregnancy.
The research, funded by Cure Kids and the Health Research Council of New Zealand, confirmed the findings from an earlier Auckland University study which was the first in the world to identify a woman's sleep position in the late stages of pregnancy as a risk factor for stillbirth.
McCowan said the findings were hugely significant because each year in New Zealand about 160 babies were stillborn in the last trimester of pregnancy and the study showed about 15 of those deaths could be prevented by changing the mother's sleep position.
"Now that we have confirmed our earlier findings, public health education encouraging women to go to sleep on their side in the last three months of pregnancy needs to be considered. This simple intervention has the potential to reduce late stillbirth by approximately 9 per cent," she said.
"Our findings make sense as lying on the back in late pregnancy is associated with physical effects that can compromise the baby's wellbeing. These include a reduction in the mother's cardiac output (the amount of blood pumped by the heart per minute), a reduced blood flow to the uterus, and lower oxygen levels in the baby".
She said her team had also been working on a study which looked at the heart rate patterns and the patterns of blood flow in babies when the mothers lay in various positions.
"We have found that when healthy women in late pregnancy lie on their back, that their babies go into a more quiet pattern of behavior that's associated with lower oxygen. So even in healthy mums we think we're seeing an effect of lying on the back so it seems to be very biologically plausible, our findings."
McCowan said the findings were particularly important because it was a risk factor that could be easily modified.
"Things like obesity and advanced maternal age are common risk factors but there's not a lot you can do about those when a woman is pregnant."
McCowan said earlier research suggested sleeping on the left side was best but to other studeis, including the the current one which saw more than 700 women spoken to, suggested that sleeping on either side was fine.
Since the university's 2009 more women had started sleeping on their side, usually on advice from a midwife. In the earlier study 43 per cent of women reported going to sleep on their side and in the current study that number had risen to 58 per cent, McCowan said.
The recent study used information from 164 women who had recently experienced a stillbirth at or beyond 28 weeks' gestation and compared that with 569 women who were pregnant with a live baby.
Ady Priday, a community midwife in Counties Manukau, said she advised her clients not to sleep on their backs and explained about the effect on blood flow to their baby. "This advice is gratefully received."
Sands New Zealand, an organisation that supports bereaved parents, also welcomed this research.
"This news about the potential to reduce stillbirth rates due to a change in sleep position is exciting. If even one more family does not have to be affected by the devastation of a stillbirth, then Sands New Zealand is supportive of these initiatives," board member Rebekah Gray said.