The study, published today in a prestigious Lancet group journal EClinicalMedicine, analysed five datasets from across the world that assessed risk factors for stillbirth.
From 28 weeks of pregnancy, sleeping while lying on the back was found to increase the risk of stillbirth by over two and a half times. This heightened risk occurred regardless of other known risk factors for stillbirth, and added to other stillbirth risk factors such as babies that are small for their gestational age.
These risks were found to be significantly reduced when women in late pregnancy slept on their side, regardless of which side the pregnant woman settled on.
"This is the definitive study of the risk of mum's sleeping position on stillbirth, and we can confidently advise mums to adopt a side-sleeping position in the last trimester knowing that they are reducing their risk," said study co-author Associate Professor Camille Raynes-Greenow from the University of Sydney's School of Public Health.
Co-author Dr Adrienne Gordon, from University of Sydney's Charles Perkins Centre and Sydney Medical School, was one of the principal investigators of the CRIBBS project (collaborative IPD in sleep and stillbirth) and led the Sydney stillbirth study which was included in the mega-study individual participant data meta-analysis.
"The study has confirmed that the risk of stillbirth associated with going to sleep on the back applies to all pregnant women in the last trimester," said Dr Gordon, who is also Deputy Chair of Red Nose Australia's National Scientific Advisory Group, and a neonatologist and Neonatal Staff Specialist with the Royal Prince Alfred Hospital's Centre for Newborn Care.
"Stillbirth is a national tragedy, which has devastating and far reaching psychosocial impacts on women, families, caregivers and communities – as well as a wide-ranging economic impact on our health system and society.
This is a huge break-through for the prevention of stillbirth, which remains one of the greatest challenges in modern obstetric practice.
Co-funded by Red Nose Australia and Cure Kids New Zealand, and led by Professor Lesley McCowan and PhD candidate and midwife Robin Cronin from the University of Auckland, the meta-analysis included 851 bereaved mothers and 2257 women with on-going pregnancies.
Professor Lesley McCowan said the increased late stillbirth risk is related to decreased blood flow to the baby by up to 80 per cent, if a pregnant woman sleeps on her back, compared to her side.
"This is due to a major vessel in the mother's abdomen, the inferior vena cava, being squashed by the womb, when a woman lies on her back," she explained.
"The mother's aorta is also partly compressed when the mother lies on her back."
Robin Cronin from the University of Auckland said: "This study shows conclusively that something as simple as going to sleep on your side can reduce the risk.
"And we know that women report they can and will change the position they go to sleep in without difficulty, if this is better for the baby."
Red Nose Chief Executive Officer Keren Ludski welcomed the publication of the research findings.
"Stillbirth is a heartbreaking and devastating tragedy that affects 2200 families in Australia every year.
"Red Nose Australia made a commitment to Australians impacted by the death of a child that we would find the answers, and research findings such as this is a huge breakthrough."
The research was an international collaboration with the University of Sydney, the University of Auckland, the University of Manchester and the University of Huddersfield in the UK, the University of Michigan in US, as well as representatives from bereaved parent groups.
