The study found that lying on the back leads unborn babies to adopt a more dormant or inactive state, possibly caused by receiving lower levels of oxygen. According to the study, “These lower oxygen levels cause the foetus to move “to a low oxygen consuming state”, while the baby’s heart rate is also affected”.
Hence, the study found that this sleeping position puts more stress on the unborn baby, causing it to reduce its oxygen intake and shedding more light on why some babies are lost late on in pregnancy. Still birth is typically defined as foetal death at or after 20 to 28 weeks of pregnancy.
It results in a baby born without signs of life. It was reported that earlier studies have shown the position mothers lie in affects their baby’s health but did not reveal how or why.
However, reacting to this development, Lead researcher in the new study, Professor Peter Stone, of the University of Auckland, New Zealand, said: “Our controlled study found that lying on your back can add extra stress to the baby, contributing to the risk of still birth.”
He noted that the risk is likely to be increased further in women with underlying health conditions. For the latest research, scientists examined 29 healthy pregnant women in the third trimester (the last three months of the pregnancy) and their unborn children.
All the women were between 35 and 38 weeks pregnant and were at low risk of complications. Each woman was asked to lie in one of four positions – lying flat, lying on their back but propped up by 30 degrees and lying on the left or right side – in a randomly allocated order.
She remained in each position with one pillow under her head for 30 minutes while researchers monitored heart rates and the activity level of the foetus. Researchers found that when women lay flat on their backs, it caused physiological stress to the baby.
In response, it switched to a sleep-like state of lower activity where it consumed less oxygen. Its heart rate was also affected. Babies in the womb were five times more likely to be in this state when mothers were lying flat than when they were lying on their left.
The study’s authors said: “Maternal position has a significant relationship with both foetal behavioural state as determined by features of foetal heart rate and its variability.
The supine position (lying face up) maybe disadvantageous for foetal well being and in compromised pregnancies may be a sufficient stressor to contribute to foetal demise.” All the women involved in the study went on to give birth to healthy babies.
Stone added: “We have only looked at the effect of maternal positions for a short period of time while the mother is awake. Further research is needed to see the effect of staying in certain maternal sleeping positions overnight.’
In its reaction, the Royal College of Midwives said the research showed “position is important” for women in their third trimester. Louise Silverton, the RCM’s director for midwifery, said:
‘We have known for a long time that blood pressure is reduced where a woman is lying on her back. Many women will say that they don’t lie on their backs as they feel “funny” when they do.
Women are advised not to sleep on their backs. Also, it is known that, when in labour, moving a woman to her left hand side may improve the foetal heart rate What this study shows that is different is, that even in the healthiest of women with a foetus with no known risk factors, position is important.
