However, the authors stress that this increased risk is still small. They explain that the absolute risk of late stillbirth for women who went to sleep on their left was 1.96 per 1,000 and was 3.93 per 1,000 for those who slept in any other position (e.g. on their back or right side).
But, they say, this finding could be important at a population level, if confirmed in other studies.
The research also finds that women who get up to go to the toilet once or less on the last night, and those who regularly sleep during the day in the last month of pregnancy, are more likely to have a stillborn baby.
The authors, led by Tomasina Stacey, PhD student, from the University of Auckland, say that stillbirth is an important public health issue which remains far too common with little change in the rate over the last 20 years.
The team interviewed 155 women in Auckland who gave birth to a stillborn baby between July 2006 and June 2009 when they were at least 28 weeks pregnant. These women were compared to a control group of 310 women with ongoing pregnancies.
Women were asked detailed questions about their sleep position (e.g. left side, right side, back or another position) on going to sleep and waking up before pregnancy and in the last month, week, and night before they believed their baby had died.
They were also asked about snoring, daytime sleepiness, whether they regularly slept during the day in the last month of pregnancy, duration of sleep at night, and how many times they got up to the toilet at night.
Demographic data, such as age, smoking status, body mass index and social deprivation level were also taken into account.
The results show no link between snoring or daytime sleepiness and risk of late stillbirth. However, a significant relation was seen between sleeping regularly during the day and late stillbirth risk, as was longer than average night time sleeping.
Women who slept on their back or on their right side on the night before stillbirth (or, for the controls, the night before the interview) were also more likely to experience a late stillbirth compared with women who slept on their left side.
Women who got up to the toilet once or less on the last night were also more likely to experience a late stillbirth compared with women who got up more frequently.
The authors suggest that restricted blood flow to the baby when the mother lies on her back or right side for long periods may help to explain this link.
They stress that this is a new observation, and confirmatory studies are needed before public health recommendations can be made. However, they conclude "if our findings are confirmed, promoting optimal sleep position in late pregnancy may have the potential to reduce the incidence of late stillbirth."
In an accompanying editorial, Dr Lucy Chappell from King's College London, says given the UK has one of the highest rates of stillbirth in the developed world "any simple intervention that reduces the risk of stillbirth would be extremely welcome".
However, Chappell argues that Stacey's research needs to be interpreted with extreme caution. She says there may be several reasons for the findings and that the results are certainly not conclusive.
Dr Chappell concludes that "a forceful campaign urging pregnant women to sleep on their left side is not yet warranted" and that at this stage, the study only puts forward a theory and more robust research is required to confirm or refute its findings.
