Women who sleep on their backs during the last three months of pregnancy are 3.7 times more likely to have a late stillbirth, Auckland University researchers say.
"Our findings make sense as lying on the back in late pregnancy is associated with physical effects that can compromise the baby's wellbeing," Professor Lesley McCowan, head of the Department of Obstetrics and Gynaecology at the university, said.
Those effects included a reduction in the amount of blood pumped by the mother's heart each minute, a reduced blood flow to the uterus, and lower oxygen levels in the baby.Sleeping on the back was also related to sleep disturbed breathing - snoring - and obstructive sleep apnoea, which have each been associated with pregnancy complications.
Public education encouraging women to go to sleep on their sides in the last three months of pregnancy needed to be considered, McCowan said.
It had the potential to reduce late stillbirth - after 28 weeks of pregnancy - by about 9 per cent, and could prevent the deaths of about 15 unborn babies a year in this country.
The study was carried out in seven district health boards - Waitemata, Auckland, Counties Manukau, Waikato, MidCentral, Capital & Coast and Canterbury - where two-thirds of New Zealand babies are born.
Researchers asked 164 women who had a stillbirth at or beyond 28 weeks' gestation questions about a range of behaviours and sleep practices, and compared that to 569 women who had live babies.
A similar study was being carried out in the United Kingdom.
The research confirms findings from a 2009 Auckland University study, which identified sleep positions in late pregnancy as a risk factor for stillbirth.
"The good news is that the position women go to sleep in can be changed.
"Since our initial findings were published in 2009, our research shows that there has been a change in going to sleep position in New Zealand women who are now more likely to go to sleep on their side," McCowan said.
"This change in behaviour has been encouraged by maternity care providers aware of the 2009 study results.
"There has also been a reduction in late stillbirths in New Zealand during this period. The reasons for this are likely to be several but could include a change in going to sleep position."
While some midwives, obstetricians and childbirth educators were already advising pregnant women not to go to sleep on their backs, a more formal campaign would be helpful so all pregnant women and maternity providers received consistent advice, she said.