Sleeping on the back during pregnancy third trimester linked to low birth weight: Study

Pregnant women who sleep on their back during the third trimester are likely to have a baby with a low birth weight, according to a new study.

This is because sleeping on the back during the third trimester has been linked to reduced blood flow to the womb, which could restrict growth, say researchers.


In the study, scientists from the University of Auckland asked 1,760 pregnant women (of mean age 30.25), who were at least 28 weeks pregnant, about their sleeping positions.

They found that the average weight of newborn babies whose mums slept on their back during the final weeks was 3.41kg, which was 144g less than 3.554kg among those whose mums went to sleep in another position, such as on their side.

This is because the lying-on-your-back-with-face-up sleeping position (also known as supine position) is associated with a reduction in maternal cardiac output and subsequent foetal blood supply.

Lying on the back towards the end of pregnancy is thought to compress the vena cava, the large vein that returns blood from the head to the heart, and the aorta, which carries blood to the rest of the body.

This reduces the maternal cardiac output (or the amount of blood a woman's heart pumps out), which in turns reduces blood flow to the foetus and placenta and lowers oxygen and nutrient supply to the foetus, the scientists wrote.

"Although sleep position changes several times during the night, women spend the longest duration in the position in which they first go to sleep. Therefore, going-to-sleep position is likely to have the greatest association with foetal blood flow and subsequent associations with birth weight," says the study.

Meanwhile, there's no need to panic if you go to sleep on your side and wake up on your back.

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"This is common - just settle back to sleep on your most comfortable side. Also, our advice is the same whether you're going to bed at night or lying down for a daytime nap," says Professor Lesley McCowan, head of the Department of Obstetrics and Gynaecology at the University of Auckland, and corresponding author of the study.

Another study had previously found that sleeping on one's back during the third trimester doubled the risk of stillbirth.

After these studies, researchers are now hoping that there will be more public health campaigns to encourage pregnant women to go to sleep lying on their side.

According to the American Pregnancy Association, the best sleep position during pregnancy is "SOS" (sleep on side). It is even better to sleep on your left side.

Sleeping on your left side will increase the amount of blood and nutrients that reach the placenta and your baby.


  • Support your back with pillows. This not only prevents you from lying on your back, it also adds comfort.
  • If you wake up in the middle of the night, make sure you haven't shifted positions and that you are still on your side.
  • Don't neglect daytime napping positions. Make sure you aren't flat on your back during daytime naps as well.
  • Don't panic when you wake up on your tummy or on your back. Just roll to your side immediately.

This article was first published in theAsianparent.