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Ask the Expert: Can sleeping on my back when pregnant harm my baby?

In late pregnancy most women find it most comfortable to sleep on their side, which is the best position for their baby

Q: I'M seven months pregnant and several people have told me not to sleep on my back as it could harm my unborn baby. Is this true? If so, why?

A: OBSTETRICIAN Professor Lesley McCowan, head of the Department of Obstetrics and Gynaecology at the University of Auckland in New Zealand, recently led a study into how babies are affected when pregnant women have slept on their backs. She says: "Fortunately, the large majority of women in the last three months of pregnancy find it most comfortable to settle to sleep lying on their side – and this is the best position for baby.

"When women in their last three months of pregnancy lie on their back, the weight of the large pregnant womb reduces the blood supply in the main abdominal vein called the vena cava. This then reduces the blood supply going from the mother's heart to the womb and can reduce the blood supply to the baby. While healthy babies cope with this reduction in flow, babies that are vulnerable for some reason may not.

"Our research has shown that pregnant women who settle to sleep lying on their backs in the last three months of pregnancy have a two to three times higher risk of stillbirth compared to mothers who go to sleep lying on their side. Our research has also shown that women who regularly go to sleep lying on their backs have babies with lower birthweight compared to mothers who go to sleep on their sides. This lower birthweight is also likely due to the reduced blood supply with back sleeping.

"The good news is that the position the mother goes to sleep in can be changed. In a survey of nearly 400 women in the last three months of pregnancy, mothers told us they could change the position they went to sleep in if it was best for baby.

"We recommend pregnant women settle to sleep on either their left or right side from 28 weeks of pregnancy. Also, it's normal to change position during sleep and many pregnant women wake up on their back. If that happens don't worry. The important thing is to start every sleep (daytime naps and going to bed at night) lying on your side and settle back to sleep on your side if you wake up."

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