Women who sleep on their back in late pregnancy are more likely to have stillbirths as the position may cut oxygen flow to the fetus, according to new research.
While the correlation between stillbirths and back sleeping isn't new information, a University of Auckland-UK collaboration now helps to explain why this is - with New Zealand's official advice to expectant mothers being "sleep on side when baby's inside".
The university, which has been working to spread the message across the world for the past decade, had women with healthy pregnancies undergo MRI scans lying on their backs and on their sides for their latest study.
The scans showed a reduction in blood flowing to the uterus when mum lay on her back, this meant that on average the amount of oxygen delivered to the fetus dropped by 6.2 percent.
"This helps us to answer questions about the rationale for the 'sleep on your side' advice," says medical student Sophie Couper who took a lead role in the research.
According to the study, healthy fetuses may be fine if mum sleeps on her back, but for vulnerable fetuses, back-sleeping may be the extra factor that causes a stillbirth.
"It seems that sleeping in the wrong position is an additional stressor that may be too much for a fetus that is already vulnerable, resulting in an increased risk of a stillbirth," says Professor Peter Stone, of the School of Medicine.
"Vulnerable fetuses include those which are smaller than usual and which may already be getting limited oxygen."
Unborn babies are less active when a mum in late pregnancy is laying on her back, this lack of activity scientists now say may be a sign there is a lack of oxygen and the fetus is changing its behaviour to conserve energy.
The Sleep on Side website estimates a 10 percent decrease in stillbirths may be achieved if all pregnant women followed the sleeping advice.
Every year in New Zealand about 160 babies are stillborn in the last three weeks of pregnancy.