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The mega-study (known as an individual participant data meta-analysis) has also confirmed that the risk of stillbirth associated with going to sleep on the back applies to all pregnant women in the last trimester.

Led by Professor Lesley McCowan and PhD candidate and midwife Robin Cronin, the research team analysed all available data world-wide from five previous studies – including the 2011 Auckland Stillbirth Study led by Professor McCowan which first identified a link between mothers’ sleeping position and stillbirth risk.

The other four studies included in the meta-analysis were a multi-centre study from Aotearoa New Zealand, one from Australia, one from the United Kingdom and one from the United States, incorporating data from women of many ethnicities.

The new study – the largest to date, including 851 bereaved mothers and 2257 women with on-going pregnancies – was funded by Cure Kids New Zealand and Red Nose Australia, and is freely available in EClinicalMedicine published by The Lancet.

The chief finding was that going to sleep lying on the back from 28 weeks of pregnancy increased the risk of stillbirth by 2.6 times. This heightened risk occurred regardless of the other known risk factors for stillbirth. However, the risk is additive, meaning that going to sleep on the back adds to other stillbirth risk factors, for example a baby who is growing poorly in the womb.

Robin Cronin

Says Ms Cronin: “Stillbirth is a tragedy for families. This study shows conclusively that something as simple as going to sleep on your side can reduce the risk. And we know that women in New Zealand report they can and will change the position they go to sleep in without difficulty, if this is better for their baby.”

Stillbirth is defined as the loss of a baby after 20 weeks of pregnancy. An estimated 2.64 million babies die before birth globally each year, and around 300 babies are stillborn in Aotearoa New Zealand each year. About one in every 500 women in New Zealand will experience a late stillbirth and lose their baby at or after 28 weeks of pregnancy.

Ms Cronin: “An important finding that came out of our study was that if every pregnant woman went to sleep on her side after 28 weeks of pregnancy, approximately six percent of late stillbirths could be prevented – around 153,000 babies a year worldwide.”

The meta-analysis was also able to reassure pregnant women that it does not matter which side they chose to settle to sleep on– both left and right sides appear equally safe.

Professor McCowan, who is the Head of the University of Auckland’s Department of Obstetrics and Gynaecology, says women should not worry if they go to sleep on their side but wake up on their back. “This is common – just settle back to sleep on your most comfortable side. Our advice is the same whether you’re going to bed at night or lying down for a daytime nap.”

Past physiological studies, carried out by the University of Auckland and funded by Cure Kids, suggested that the increased late stillbirth risk is due to a major vessel in the mother’s abdomen (the inferior vena cava) being squashed by the pregnant womb when she is lying on her back, which reduces flow by 80 percent. Although the mother’s circulation responds by increasing the flow through other veins, this does not fully compensate. The mother’s aorta – the main artery which carries oxygen-rich blood from her heart – is also partly compressed when the mother lies on her back, decreasing blood flow to the baby.

Professor McCowan: “While healthy unborn babies can compensate for the reduced blood supply, babies that are unwell or vulnerable for some other reason, may not cope.”

The other researchers in this international collaboration are from the University of Sydney Australia; the University of Manchester and the University of Huddersfield in the UK; and the University of Michigan, US as well as representatives from bereaved parent groups.

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