Researchers can't say for certain why the risk of stillbirth is increased - but there is a lot of data that suggests when a woman is lying on her back, the combined weight of the baby and womb puts pressure on blood vessels which can then restrict blood flow and oxygen to the baby.

Edward Morris, from the BJOG, said the new research was "extremely welcome" .

"This is an important study which adds to the growing body of evidence that sleep position in late pregnancy is a modifiable risk factor for stillbirth."

The pregnancy charity Tommy's has started a campaign to raise awareness of the study and to encourage women to sleep on their side.

Michelle Cottle's baby Orla was stillborn at 37 weeks in 2016 after a healthy pregnancy where there were no signs that anything was wrong.

She writes a blog "Dear Orla" and hears from women who have been through the same experience.

Michelle, whose daughter Esme was born a year later, says practical advice like this for mothers is important to make them feel more in control.

"I really think it helps to empower people, as it feels like something you can go away and do with the hope of having a more healthy pregnancy and a better outcome than sadly lots of people do have.

"I look back now and I actually feel quite traumatised by my (second) pregnancy because it was a bit like living your worst nightmare every single day.

"Every time there's maybe a quiet moment and you don't know whether your baby is alive or not is absolutely terrifying.

"Night-time is the worst as well because a lot of people would say they believe that their baby died maybe when they were sleeping. I think that's really scary because you have to sleep.

"So I think having clear things that can help you feel a bit more in control is really important for women."