Researchers interviewed 155 women in New Zealand who gave birth to a stillborn baby between 2006 and 2009 when they were at least 28 weeks pregnant. They were compared to 310 other women with ongoing pregnancies.
The chance of a baby being stillborn rose to nearly 4 in 1,000 for those who don't sleep on their left side, compared to nearly 2 in 1,000 for those who do.
Scientists say it may be due to restricted blood flow, but more research is necessary.
The study was published in the British Medical Journal.