What people believe
The advice to sleep exclusively on the left side during pregnancy is one of the most widely repeated and anxiety-inducing recommendations. Many pregnant women lie awake worrying about rolling onto their back or right side, or feel they have done something dangerous when they wake up in a different position.
What the evidence says
The concern is specifically about going to sleep on your back (supine position) in the third trimester — not about side preference. A 2019 individual participant data meta-analysis (Cronin et al., eClinicalMedicine/The Lancet) found that supine going-to-sleep position after 28 weeks was associated with a 2.6-fold increased risk of late stillbirth.1
The critical finding: going to sleep on the left side or the right side appeared equally safe.1 There is no evidence that right-side sleeping carries increased risk compared to left-side sleeping. The risk is specific to the supine (back) position.
This applies to the position you fall asleep in, which is the position held longest during the night. If you wake up on your back, simply roll to either side.3 The evidence applies after 28 weeks, not throughout pregnancy.
What to do instead
After 28 weeks, settle to sleep on either side. Left or right are equally safe. If you wake on your back, roll to a side and go back to sleep. Using a pillow behind your back can help you stay on your side. Do not lose sleep worrying about sleep position — the stress and sleep deprivation from anxiety are themselves harmful.