Stillbirth risk more than doubles if women go to sleep on their backs in the last three months of pregnancy, according to new research.
A large-scale study of pregnant women in England found a 2.3 fold increase in the risk of late stillbirth among those who went to sleep on their backs in the third trimester.
Meanwhile the study estimates there would be a 3.7% decrease in stillbirth if all women in the UK went to sleep on their sides in the final months before childbirth.
Pregnancy support charity Tommy's has launched the Sleep on Side campaign to raise awareness among women of the risks of going to sleep on their backs for any kind of sleep late in pregnancy.
The charity advises women to go to sleep on their sides, but not be concerned if they wake up on their back as the position at falling asleep is usually held longest during the night.
Should they wake in the middle of the night they should roll on to their sides before going back to sleep, the charity says.
Around one in every 224 pregnancies in the UK ends in stillbirth and the Midlands and North of England Stillbirth Study (MiNESS) was the largest of its kind to examine the impact of maternal sleep position on the baby loss.
Professor Alexander Heazell, clinical director at the Tommy's Stillbirth Research Centre at St Mary's Hospital, Manchester, led the study.
He said: "Around 11 babies are stillborn every day in the UK. Stillbirth is devastating, with long-lasting effects on bereaved parents.
"Parents want to know why their baby has died, whether it might happen again if they try for another baby and what they can do to avoid further stillbirth."
How sleep position affects stillbirth risk is yet to be established, although it is thought the weight of the baby and uterus can impair blood flow to the baby, or the sleep position can affect breathing.
Louise Silverton, director for midwifery at the Royal College of Midwives, said: "Stillbirth is a terrible tragedy for mothers and their families and we must do all we can to bring stillbirth rates down.
"This addition to current knowledge is very welcome. The Tommy's campaign and the research findings are a great example of how through making small changes we can begin to bring down stillbirth rates.
"It is a simple change that can make a difference and it will be important to ensure that this is communicated effectively to women."
Edward Morris, vice president for clinical quality at the Royal College of Obstetricians and Gynaecologists (RCOG), said the study was "extremely welcome as a significant number of stillbirths remain unexplained, particularly those in late pregnancy".
"The impact of stillbirth on parents and professionals is devastating and the RCOG is committed to working collaboratively on research, audit and training for healthcare professionals in order to achieve a substantial reduction in the UK stillbirth rate."
MiNESS, published in the British Journal of Obstetrics and Gynaecology, follows similar research in New Zealand and Australia that found an increase in stillbirth if women sleep on their back in the third trimester.