Pregnancy and child

Pregnant women 'should avoid sleeping on back in last trimester'

"New warning to pregnant women: Do not sleep on your back in the last trimester as it could cause stillbirth, claim experts," the Mail Online reports.

This rather overdramatic headline stems from a new study that investigated the effects of mothers' sleep positions on baby behaviour in 29 women in the final weeks of pregnancy.

Compared with when mothers slept on their left side, which was most common, babies were slightly more likely to be active and awake when women slept on their right side, and slightly more likely to be quietly asleep when women slept on their back.

But the differences in the babies' activity patterns were very small.

Changes in maternal position and a baby's activity pattern naturally altered the baby's heart rate pattern, but all the babies were born completely healthy.

On its own, this research doesn't provide any evidence that the position a mother sleeps in may harm her baby.

But previous research has suggested that sleeping on your back when you're pregnant may increase the risk of stillbirth, as it compresses the mother's major blood vessels and alters the baby's heart rate.

For this reason, the study's authors suggest women avoid sleeping on their backs in the last trimester of pregnancy.

Where did the story come from?

The study was carried out by researchers from the University of Auckland in New Zealand.

Joint funding was received from the children's charity Cure Kids and the University of Auckland.

The study has been peer-reviewed and accepted for publication in The Journal of Physiology, but hasn't been formally published yet.

It's available to read free online as an accepted article, but there may be some changes in the production of the final draft.

Both the Mail Online and the Daily Mirror talk about an increase in the risk of stillbirths from a pregnant woman sleeping on her back.

The researchers didn't investigate this, and all the babies involved in this study were born healthy.

The main body of the media articles did, however, provide a more accurate representation of the findings.

What kind of research was this?

This observational study aimed to investigate the effects of pregnant women's sleep positions on foetal behaviour late in the third trimester.

The third trimester starts from 29 weeks and continues to the end of pregnancy.

The researchers wanted to assess the effects in as natural a setting as possible. Women wore foetal monitors while they slept at home and weren't advised what position to sleep in.

Observational studies are useful for testing the link between a possible exposure and outcome – in this case, the mother's sleep position and foetal behaviour – but can't confirm cause and effect.

Although a randomised controlled trial (RCT) would be the most ideal way to test an association, it wouldn't be ethical to make pregnant women sleep in positions that may risk harming their babies.

What did the research involve?

The study recruited 29 healthy pregnant women carrying a single foetus who were late into their third trimester (36 to 38 weeks).

All women were told to sleep as they would normally, and the researchers set up recording equipment to study the participants in their own homes.

Video footage was collected to determine maternal sleeping position.

The onset of sleep was defined as the first three minutes during which there were no movements.

Position changes were counted as positions that were assumed for longer than three minutes.

Sleep positions through the night were categorised as:

  • left lateral (left side)
  • right lateral (right side)
  • supine (back)

A continuous foetal echocardiogram (ECG) was used to record maternal and foetal heart rate.

Mean foetal heart rate was assessed for every minute from when the mother fell asleep until she woke up. Consistent states were defined upon duration of three minutes.

Foetal behavioural states were determined using the following:

  • 1F – quiet sleep
  • 2F – active sleep
  • 3F – quiet awake (rarely seen in foetuses)
  • 4F – active awake

Researchers analysed the relationship between maternal position and foetal state.

What were the basic results?

The average maternal sleep duration was approximately eight hours. Sleeping on the left side was the dominant position in the majority of women.

  • In all maternal sleeping positions, the foetuses were in a state of active sleep (2F) more than 80% of the time. They were in quiet sleep (1F) 13% of the time and spent little time actively awake.
  • Foetal heart rates were lower in the state of quiet sleep (1F) than when actively asleep (2F), and higher when actively awake (4F).
  • State 4F was found to be more likely earlier in the night compared with state 1F, which was more likely later into the night.

Compared with the mother sleeping on her left side:

  • Quiet foetal sleep (1F) was more common when the mother slept on her back (odds ratio [OR] 1.30, 95% confidence interval [CI]: 1.11 to 1.52) and less common when she slept on her right side (OR 0.81, 95% CI: 0.70 to 0.93). Although statistically significant, the actual difference in the amount of time the babies spent in this state was small (13.4% when mothers were on the left side versus 14% on the back and 11.3% on the right).
  • The foetus was less likely to be actively awake (4F) when the mother slept on her back (OR 0.33, 95% CI 0.21 to 0.52) and more likely when she slept on her right (OR 1.72, 95% CI 1.37 to 2.18). But the actual difference between sides was very small: 0.8% of foetal time when on the back compared with 4.4% of time on the left and 5.2% on the right.

All babies were healthy at their six-week postnatal check-up.

How did the researchers interpret the results?

The researchers said: "Our results have shown that time of night significantly influenced the likelihood of the foetus being in a particular state, with 4F being more likely in the early part of the night and 1F less likely then and more likely later after sleep onset.

"This may be due in part to the maternal position effects where position change, most often from non-supine to supine sleep, occurred after the period of most stable sleep.

"It was also found that the effects of foetal state on measures of foetal heart rate variability were modified by maternal position, likely mediated through autonomic nervous system activity.

"This further supports the concept that maternal position is an important modulator of circadian effects on foetal heart rate."


This observational study suggests a mother's sleep position may influence their baby's activity in late pregnancy.

Most mothers sleep on their left side, but babies were found to be slightly more likely to be actively awake if women slept on their right side.

If they slept on their backs, babies were slightly more likely to be quietly asleep.

These are interesting findings, but there are a few points to note:

  • In all maternal sleeping positions, the foetuses were in a state of active sleep more than 80% of the time. Although there was a statistically significant difference in the amount of time babies spent quietly asleep or actively awake during different maternal sleeping positions, the actual percentage difference was only very small (less than 5% difference in all cases).
  • This was a very small study – a much larger sample size of mothers would be needed to validate these findings.
  • There may be other factors in play, such as mothers' diet and physical activity during the day.
  • The baby's sleep or activity patterns automatically influence their heart rate.
  • All the babies were born healthy. There's no evidence that sleep position puts the baby at risk of harm.

Some organisations, such as the American Pregnancy Association, recommend pregnant women sleep on their left side as this will "increase the amount of blood and nutrients that reach the placenta".

NHS Attribution