Side-sleeping in late pregnancy best for baby’s growth

A University of Auckland-led study has found for the first time that women who go to sleep on their back over the last one to four weeks have a baby that is, on average, 144g lighter than women who settle on their sides – equivalent to the effect of smoking 10 cigarettes daily.

No need to panic, though, if you go to sleep on your side but wake up on your back, the researchers say. “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 senior author Professor Lesley McCowan, who is the Head of the University’s Department of Obstetrics and Gynaecology.

When it comes to newborns, being too light is more perilous than being too heavy. Babies who are small for their gestational age (SGA), defined as having a birthweight in the lowest 10 percent of babies born, are more likely to be stillborn or have health problems before and following birth.  

Earlier this year, the same research team published a meta-analysis, or ‘mega-study of studies’, showing that women in the third trimester of pregnancy who go to sleep lying on their backs are two to three times more likely to have a stillborn baby, compared with women who do not go to sleep lying on their backs.

The likely reason for this link is that lying on the back in late pregnancy reduces blood supply to the placenta and baby due to a major vessel in the mother’s abdomen (the inferior vena cava) being compressed by the size of the large pregnant womb.  

Knowing that another birth complication, fetal growth restriction, is also associated with less blood getting to the fetus, the researchers wanted to see if there was a link between back-sleeping and lower birth weight. So they analysed the data from the women in the ‘control’ groups of the stillbirth studies from Aotearoa New Zealand, Australia and the United Kingdom.

Of the 1760 pregnant women who were interviewed from 28 weeks’ gestation or later, 57 (3.2 percent) reported that they usually went to sleep on their back during the previous one to four weeks. These women were three times more likely to have a SGA baby, and the babies were on average 144g lighter, with a 10 percent decrease in birthweight percentile.

Reassuringly, there was no increase in the proportion of babies who were born heavy (>90th centile) in mothers who went to sleep lying on their side.

“This reduction in birth weight with back sleeping could partly explain the relationship we have seen between back sleeping and elevated risk of stillbirth,” says lead author Dr Ngaire Anderson, a senior lecturer in Obstetrics and Gynaecology in the University of Auckland’s Faculty of Medical and Health Sciences and obstetrician and gynaecologist at Waitematā DHB.

“The good news is women can change their going-to-sleep position.”

Professor McCowan says the findings reinforce the importance of an ongoing public health campaign that reminds women to ‘sleep on side when baby’s inside’ to reduce the risk of stillbirth. Going to sleep on their side may have the added benefit of optimising the birthweight of their baby.

The research was funded by Cure Kids New Zealand and Red Nose Australia. The findings are published in the medical journal JAMA Network Open.

Read the article:

JAMA Network Open: Association of supine going-to-sleep position in late pregnancy with reduced birth weight – A secondary analysis of an individual participant data meta-analysis