Pregnant women have another reason to sleep on their side in their final trimester, with a study showing it’s the best position for the baby’s growth.
The University of Auckland-led research has found a baby is, on average, 144 grams lighter when expecting mothers sleep on their back rather than their side, in the last 1-4 weeks of pregnancy.
The weight difference is equivalent to the effect of smoking 10 cigarettes daily.
“Although it doesn’t sound like a lot, actually the impact is potentially large”, said lead author Ngaire Anderson.
The research team have determined this using data from their earlier study, which focussed on stillbirths.
That study found 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.
Dr Anderson said, “The reality is, a very small proportion of women find it comfortable to lie on their back in the later part of pregnancy, so in our study, only about three per cent of women found it comfortable”.
These women were three times more likely to have a baby small for its gestational age.
Babies who are small for their gestational age, defined as having a birthweight in the lowest 10 per cent of babies born, are not only more likely to be stillborn but also are more likely to have health problems before and following birth.
Dr Anderson said this could include the babies being more stressed in labour, or needing breathing support after birth.
“I think every mother would agree optimising their baby’s growth is a priority”, she said.
There was no increase in the proportion of babies who were born heavy in mothers who went to sleep lying on their side.
Researchers say there’s no need for women to panic if they go to sleep on their side but wake up on their back.
Professor Lesley McCowan, a senior author of the report, said it is common.
"Just settle back to sleep on your most comfortable side," she said. "Also, our advice is the same whether you’re going to bed at night or lying down for a daytime nap.”