Is sleeping on your back safe during pregnancy?

The verdict

In the UK, no, not after 28 weeks, and this is now formal national guidance rather than advice from a charity.

NICE's antenatal care guideline NG201 contains two recommendations on the subject. The first: advise women to avoid going to sleep on their back after 28 weeks of pregnancy and to consider using pillows, for example, to maintain their position while sleeping. The second: explain to the woman that there may be a link between going to sleep on her back and stillbirth in late pregnancy, after 28 weeks.

The wording of the second recommendation is worth reading carefully. NICE says there may be a link. It does not say that sleeping on your back causes stillbirth, and that restraint reflects what the evidence can and cannot support.

What the studies actually showed

The evidence is observational, and all of it comes from case-control studies, meaning studies that ask women who have had a stillbirth about their sleep position and compare their answers with women who have not.

Tommy's, which ran the UK Sleep On Side campaign and funded the research, summarises the position: the link has been shown in six separate research studies, and the data from those studies has been combined for more detailed analysis that confirms going to sleep on your back is linked with a 2.6 times higher risk of stillbirth, as well as a higher risk of having a baby that is smaller than expected. The pooled analysis is an individual participant data meta-analysis published in EClinicalMedicine in 2019.

Three limitations follow from that design, and none of them is a reason to ignore the advice.

First, case-control studies establish association, not causation. Second, they rely on recall, and asking a woman who has just had a stillbirth to remember how she fell asleep is an unavoidably imperfect measurement. Third, a relative risk means little without the absolute one. Tommy's supplies it: in an uncomplicated pregnancy the risk of stillbirth is low, around 3 in 1,000 babies in the UK, and going to sleep on your side makes it lower still.

There is a plausible mechanism, which is part of why the finding has been taken seriously. Lying on your back in late pregnancy puts the combined weight of the uterus and baby onto the major blood vessels behind it, which can restrict blood flow. Tommy's notes that studies have shown that when a woman lies on her back in late pregnancy, compared with lying on her side, the baby is less active and shows changes in heart rate patterns, thought to be due to lower oxygen levels.

Left side or right side?

Either. This is where a great deal of internet advice goes beyond the evidence.

Tommy's addresses it directly: many websites say the left side is best, but this is not supported by good research evidence. One smaller study from Auckland found that women who slept on their left side on the last night of pregnancy halved their risk compared with those on their right, but the same finding has not been seen in any of the three published studies since. Sleeping on your left has not conclusively been shown to reduce risk compared with sleeping on your right.

Either side. That is the whole answer, and it removes a lot of unnecessary anxiety.

What if you wake up on your back?

Roll over and go back to sleep. This is the single most reassuring point in the guidance and the one that gets lost.

Tommy's explains that the research focused on the position women went to sleep in, not the position they were in during the night, and that we cannot control our position while asleep. The reason the going-to-sleep position was studied is that it is the position we spend the longest in overnight. A large bump is also usually uncomfortable enough to prevent you staying on your back for long.

The advice applies to every episode of sleep, not just bedtime: going to sleep at night, returning to sleep after waking in the night, and daytime naps.

Practical ways to stay on your side

Tommy's suggests putting pillows behind you so that rolling onto your back is uncomfortable, noting that regular pillows and pregnancy pillows are equally effective. It also offers an unexpectedly practical tip: tying long hair in a low bun makes lying on your back uncomfortable. And if you have pelvic girdle pain, which makes side-lying hard, a pillow under the bump and between the legs helps, keeping the knees together when turning over reduces the pain, and a physiotherapy referral is worth asking your midwife for.

Sleeping on your front is not a concern. Tommy's notes it is fine early in pregnancy and that by the third trimester a large bump makes it very unlikely you would choose it anyway; if you wake up on your stomach, just roll onto your side.

Lying on your back when you are awake

This is a separate question with a separate and earlier threshold, and it applies whether or not you are asleep.

The NHS advises not lying flat on your back for long periods, particularly after 16 weeks, because the weight of your bump presses on the main blood vessel bringing blood back to your heart and this can make you feel faint. ACOG gives the same advice in its exercise guidance: avoid standing still or lying flat on your back as much as possible, because the uterus presses on a large vein that returns blood to the heart, and these positions may cause blood pressure to decrease for a short time.

That is the reason exercise classes, massage tables and scans in later pregnancy tilt you or prop you on one side. Brief periods on your back for a medical examination are fine, and if you feel faint, telling the person examining you is the correct response.

Where guidance genuinely differs

The UK has a formal recommendation; the US does not. NICE NG201 recommendations 1.3.25 and 1.3.26 make going to sleep on your side after 28 weeks part of routine antenatal advice in England and Wales, supported by a dedicated evidence review. ACOG publishes no equivalent recommendation about going-to-sleep position and stillbirth. Its advice about lying flat on your back appears in exercise guidance and is about faintness and blood pressure, not about stillbirth risk.

The advice is recent, and it changed. Sleep position was not part of standard UK antenatal advice until the last few years; NG201 was published in 2021. Anyone who was pregnant before then was not given this advice, and that is not a reason for retrospective worry.

The research is largely from the UK, New Zealand and Australia. The Auckland study and the pooled individual participant data analysis came out of that group of countries, which is part of why the guidance has been adopted there and not in the United States. It is a difference in how the same observational evidence has been weighed, not a disagreement about what the studies found.