Pregnant women should sleep on their side as those who slumber on their backs are more than twice as likely to suffer a stillbirth, say scientists.
A study of more than 1,000 expectant mothers in England - the largest of its kind - found doing so raised the risk an alarming 2.3 times.
It estimates there would be 3.7 per cent fewer stillbirths if all pregnant women in the UK slept on their sides during the last three months of their term.
It's believed when pregnant mothers sleep on their backs it reduces the flow of oxygen and blood to the baby once it reaches a certain weight inside the womb.
The finding adds to previous evidence - based on much smaller numbers - of a link between sleep position in the final trimester and death of the unborn infant.
Researcher Dr Tomasina Stacey, lecturer in maternal health at Leeds University, said: "A stillbirth is a devastating experience.
"The message that emerges from this research though is that women can modify this particular risk factor themselves.
"When they go to bed, they should try to settle to sleep on their side and not their back, it doesn't matter if it's the left side or the right side.
"They should not worry if they wake up and find themselves on their back. The important thing is to commence sleep on their side."
High UK figures
In the UK around 11 babies or about one in 225 are stillborn every day ranking it the 24th out of 49 high-income countries.
The international team which also included colleagues at Manchester University said
the UK's comparatively high rate shows the situation should be improved.
Dr Stacey said: "The research has shown women who settle to sleep on their back on their last night of pregnancy appear to have a considerably raised risk or likelihood of stillbirth greater than twofold.
"We don't know for sure what's going on. One of the reasons may be the weight of the baby and uterus on the main arteries in the body reduces the blood flow back to the mother's heart and eventually reduces it to the baby by about 30 per cent."
She said the extra strain could be a "tipping point" for babies already vulnerable to stillbirth by being weakened by other factors.
The study published in the Journal of Obstetrics and Gynaecology recruited participants from 41 maternity units in the Midlands and North of England - 291 of whom had a stillbirth 28 weeks or later into their pregnancy.
They were compared to 735 women who had a live birth and acted as a control group.
The women were interviewed about their sleeping positions amongst a number of other factors and identified the sleep position phenomenon on the night before their baby died.
The researchers estimate that if no pregnant women slept on her back, late-stage stillbirths could fall by 3.7 per cent in England.
The study did not investigate how sleeping positions were impacting on the unborn baby but the researchers suggest a number of theories.
In the late stage of pregnancy the combined weight of the baby and the womb puts pressure on the mother's blood flow when she is lying on her back.
This can restrict blood flow and oxygen to the baby. Also, a woman may experience disturbed breathing patterns when she is asleep on her back.
Dr Stacey was one of the first academics to highlight the association between sleep position and the increased risk of stillbirth in a paper that was published in The BMJ in 2011.
Then working in New Zealand she said more research was needed to confirm her findings.
Since then there have been three other studies with similar results. This latest research is the largest investigation of the observation to date.
Lead author Professor Alexander Heazell from Tommy's Stillbirth Research Centre at St Mary's Hospital in Manchester, said: "Around 11 babies are stillborn every day in the UK.
"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."
Tommy's - which funded the research with three other charities including Action Medical Research, Cure Kids and Sands - is launching a public health campaign to alert women it's safer for their baby to sleep on their side.
Louise Silverton, director for midwifery at the Royal College of Midwives, said: "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's a simple change that can make a difference and it will be important to ensure this is communicated effectively to women."
The study also found stillbirths are also more likely to occur if women sleep less than 5.5 hours the night before, get up in the night to use the toilet or have a nap every day.
Sleeping on your back while pregnant does not influence a baby's size or the length of a woman's gestation.
Previous research reveals pregnant women's sleeping positions have a significant effect on their babies' heart rate variability.
When such women sleep on their backs, their babies are less active than when they nod off on their side.
If mothers-to-be change their position during sleep, for instance from lying on their left to their back, the baby quickly becomes 'quieter'.