Women who sleep on their back during their final weeks of pregnancy are likely to have a baby with a lower birth weight - equivalent to the effect of smoking 10 cigarettes a day.
That's according to a University of Auckland-led study involving 1760 pregnant women from New Zealand, Australia and the United Kingdom who were interviewed from 28 weeks gestation.
The international research, funded by Cure Kids New Zealand and Red Nose Australia, found women sleeping on their back in the last one to four weeks of pregnancy were three times more likely to have a small for their gestational age (SGA) baby, which was 144g lighter than the average.
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Researchers said this was likely because lying on the back in late pregnancy reduces blood supply to the placenta and baby due to a major vessel in the mother's abdomen being compressed by the size of the large pregnant womb.
Babies who were SGA, which had a birth weight in the lowest 10 per cent of babies born, were likely to be stillborn or have health problems before and following birth.
For expectant mum Amy Marshall these findings were no surprise because the link between sleeping on your back and stillbirths was already well-known.
"I guess it just reinforces the importance of sleeping on your side."
Marshall, who is seven months pregnant with her second child, said she had an elaborate pillow set-up to ensure she didn't roll on her back.
"It's probably not ideal for the person sleeping next to you but it's worth it in the end," Marshall said.
She said it was alarming to know sleeping on your back had as much risk as smoking 10 cigarettes a day.
"That's pretty scary.
"There has been a few occasions where I have woken up on my back but I usually just roll back on my side," Marshall said.
Professor Lesley McCowan, head of the Department of Obstetrics and Gynaecology at the University of Auckland, said there was no need to panic if you go to sleep on your side and wake up on your back.
"This is common - just settle back to sleep on your most comfortable side. Also, our advice is the same whether you're going to bed at night or lying down for a daytime nap."
Of the total 1760 expectant mothers included in the study, 57 (3.2 per cent) said they had tended to sleep on their back during the past one to four weeks.
These women were three times more likely to have a SGA baby.
McCowan said the good news was women could change their going-to-sleep position.
"The findings reinforce the importance of an ongoing public health campaign that reminds women to 'sleep on side when baby's inside' to reduce the risk of stillbirth.
Going to sleep on their side may have the added benefit of optimising the birth weight of their baby, McCowan said.
The same research group previously found women in the third trimester of pregnancy who went to sleep lying on their backs were two to three times more likely to have a stillborn baby, compared with women who did not go to sleep lying on their backs.
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.