Pregnant women who go to sleep on their back are almost four times more likely to have a stillbirth, Auckland University researchers have found.
New Zealand Multi-Centre Stillbirth Study], funded by Cure Kids and the Health Research Council, found a 3.7-fold increase in stillbirth among mothers who slept on their backs in the last three months of pregnancy.
It suggested 15 unborn babies' lives could be saved a year if the message about sleep position was well promoted.
The study confirmed earlier Auckland University research, which was the first internationally to identify maternal sleeping position as a risk factor in stillbirths after 28 weeks of pregnancy, independent of other common stillbirth risk factors.
Head of Obstetrics and Gynaecology Lesley McCowan, who led the study, said lying on the back could reduce the amount of blood pumped by the heart, reduce blood flow to the uterus and lower oxygen levels in babies.
Researchers found that the risk of a stillbirth associated with going to sleep on the back was higher in term pregnancies - meaning after 37 weeks - compared to pregnancies between 28 and 36 weeks.
About 160 babies are stillborn in the last trimester in New Zealand each year.
Professor McCowan said some midwives, obstetricians and childbirth educators had been advising pregnant women not to go to sleep on their backs since the 2009 research, and there had been a reduction in stillbirths since then.
"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," she said.
"This simple intervention has the potential to reduce late stillbirth by approximately 9 percent."
She said sleeping on the back was also related to sleep-disturbed breathing and obstructive sleep apnea, which have each been associated with pregnancy complications.
The research was conducted in seven New Zealand district health board areas, where two thirds of all New Zealand births take place: Waitemata, Auckland, Counties Manukau, Waikato, MidCentral, Capital & Coast and Canterbury.
In a case-control study, 164 women experienced a stillbirth at or beyond 28 weeks' gestation in their current pregnancy and 569 remained pregnant, allowing comparisons to be made between mothers with stillborn infants and those with ongoing pregnancies.
Participants were asked about a range of behaviours and sleep practices including what position they slept in.
The study was funded through a partnership grant between Cure Kids and the Health Research Council of New Zealand.
Cure Kids also funded the 2009 Auckland Stillbirth Study and is co-funding a similar study in the United Kingdom, which some of the New Zealand research team are contributing to.
Sands New Zealand, a voluntary parent-run organisation that supports bereaved parents, welcomed the research.
Sands board member Rebekah Gray said stillbirth was one of the most traumatic experiences parents could go through.
"The shock and grief for families cannot be imagined. This news about the potential to reduce stillbirth rates due to a change in sleep position is exciting," she said.
Professor Ed Mitchell, who had a leadership role in the study, said he hoped it would have a similar impact as the 'Back to Sleep' campaign for reducing Sudden Infant Death Syndrome, which started in the late '80s after research he led.
Researchers said pregnant women could use pillows to stay sleeping on their side, but stressed there was no need to worry if they found they had rolled onto their back during the night, as the most important position was the one they fell asleep in.
They said many pregnancy websites suggested that sleeping on the left side was best, but studies were divided on whether there was any specific benefit and recommended only that pregnant women fall asleep on their side.