Women who sleep on their back during pregnancy may raise the risk of miscarriage or stillbirth, a five-year Australian study has found.
The research titled the "Sydney Stillbirth Study" looked at pregnancies of 295 women from eight hospitals around Australia, the Daily Mail reported.
The study found that women who sleep on their back are six times more likely to have a stillborn baby.
A stillborn baby is a baby born dead after 24 weeks of pregnancy. If the baby dies before 24 completed weeks, it is known as a late miscarriage.
Lead researcher Adrienne Gordon, from Sydney's Royal Prince Alfred Hospital, said previous research suggested prolonged periods of lying in this position restricted blood flow to the baby.
It is believed that sleeping on the right side or on the back reduces blood flow through a major vein from the legs to the heart, which affects the supply to the womb, the study said.
However, the researchers said it was important that women who are currently pregnant "don't become alarmed if they sometimes sleep on their back".
Experts said three-quarters of pregnant women sleep mostly on the left side.
The daily said this may suggest they instinctively choose a sleeping position that works best for the baby.
In almost half of stillbirths, the direct cause of the baby's death cannot be established.
Ten percent of stillborn babies have some kind of abnormality and other possible causes include problems with the mother's health or problems with the placenta, which links the baby's blood supply to that of the mother.
Stillbirth Foundation Australia, which funded the study, said the research was unique as it looked exclusively at women who were more than 32 weeks pregnant.
In 2011, a University of Auckland study found that mothers who slept on their back or right-hand side on the night before giving birth were twice as likely to have a stillborn child compared to those who slept on their left.