Researchers suggest lying flat on the back in later pregnancy may raise the risk of stillbirth.
Stillbirth - defined as the death of a baby after 20 weeks of pregnancy - is estimated to affect around 1 percent of pregnancies in the United States, with around 24,000 babies being stillborn each year.
According to the Centers for Disease Control and Prevention (CDC), the causes of many stillbirths are unclear, but they generally include birth defects, problems with the umbilical cord or placenta, and maternal health issues.
Now, Peter Stone, professor of maternal fetal medicine at the University of Auckland in New Zealand, and colleagues suggest maternal position in later pregnancy may affect the risk of stillbirth.
When it comes to sleeping positions in pregnancy, expectant mothers are already warned to avoid sleeping on their backs. According to the American Pregnancy Association, such a position can reduce blood circulation to the mother's heart and to the baby.
For their study - published in The Journal of Physiology - Prof. Stone and colleagues further investigated the effects of maternal positions on fetal health.
Lying on the back lowers baby's oxygen supply
The researchers enrolled 29 healthy women who were in the third trimester of pregnancy.
The women were asked to lie down in a variety of positions for 30 minutes at a time, and the researchers monitored the health of both the mother and fetus in each position.
All women were followed until they gave birth, and all babies were healthy when born. Still, the team uncovered some concerning information about the effects of maternal positions on fetal health.
The team found that when the mothers laid on their backs, the position altered the baby's heart rate and activity, causing a reduction in oxygen supply. This can increase the risk of stillbirth.
"Our hypothesis is that lying flat on the back compresses the main veins bringing blood back to the mother's heart - an effect well described," Prof. Stone told Medical News Today, "but then the mother's cardiac output may fall and then the flow to the womb is reduced. We suspect that many mothers and babies can cope, but some do not."
Evidence suggests mothers should lie on the left side
Prof. Stone told MNT that the researchers were initially surprised by the findings; they did not expect to find altered fetal activity with such short periods of lying down, particularly in healthy pregnancies.
"Thus, we were surprised about how even a normal baby made adaptations quite quickly to what was likely to be reduced placental blood flow and therefore oxygen in that position," he added.
For mothers with underlying health conditions, the researchers hypothesize that the risk of stillbirth from lying flat on the back may be even higher.
Based on this research and findings from previous studies, the researchers suggest expectant mothers should be cautious about what positions they lie in.
"[...] we feel there is sufficient evidence to say that lying flat on the back is disadvantageous to the unborn baby in the later parts of pregnancy, so we do suggest avoiding that. We think the evidence is in favor of lying on the mother's left side."
Prof. Peter Stone
Because the researchers only studied each maternal position for 30 minutes, they are unable to say how certain positions might affect fetal health overnight during sleep. This is something they plan to investigate in future research.
Additionally, the researchers want to learn more about the underlying mechanisms by which lying on the back during pregnancy affects fetal health.