Sleep position in late pregnancy has been the subject of intense research interest since the early 2010s, when observational studies first identified an association between supine (back) sleeping and adverse outcomes. The evidence has since matured. Here is what it shows.
The key evidence
The most authoritative evidence comes from an individual participant data (IPD) meta-analysis published in eClinicalMedicine (The Lancet) in 2019 by Cronin and colleagues. This study pooled data from five case-control studies across four countries (New Zealand, Australia, UK, US), covering 851 cases of late stillbirth and 2,257 controls.1
The findings: women who reported going to sleep in a supine position after 28 weeks' gestation had a 2.6-fold increased risk of late stillbirth compared to women who went to sleep on their side (adjusted OR 2.63, 95% CI 1.72–4.04). The association held after adjusting for known stillbirth risk factors including maternal BMI, age, parity, and smoking status.1
Importantly, the finding relates to going-to-sleep position — the position you settle into when you fall asleep — not the position you wake up in. Research cannot control for movements during sleep, and the studies did not find that waking up on your back was associated with the same risk. If you fall asleep on your side and wake up on your back, the evidence does not suggest you need to be alarmed.2
The proposed mechanism
The biological explanation centres on aortocaval compression. In the supine position, the weight of the pregnant uterus compresses the inferior vena cava and aorta, reducing blood return to the heart and decreasing cardiac output and uterine perfusion. In the third trimester, when the uterus is at its heaviest, this compression can significantly reduce blood flow to the placenta.3
Left lateral sleeping is considered optimal because it moves the uterus away from the vena cava and allows maximum blood flow to the placenta. However, the IPD meta-analysis found that either left or right side sleeping was protective — the critical distinction is between any lateral position and supine, not between left and right specifically.1
What the guidelines say
Following the IPD meta-analysis, public health campaigns in multiple countries now recommend that pregnant women go to sleep on their side from 28 weeks onwards. NHS and Tommy's charity in the UK launched the Sleep On Side campaign. NSW Health in Australia published specific guidance. ACOG has acknowledged the research while noting the observational nature of the evidence.
The guidance is consistent: go to sleep on your side from the third trimester. Either side is fine. If you wake up on your back, simply roll over — the evidence relates to the position in which you settle to sleep, not every position adopted during the night.2
Practical strategies
Place a pillow behind your back to prevent rolling onto your back during sleep. A pregnancy pillow or rolled towel can serve the same function. If you find side sleeping uncomfortable, a pillow between your knees and under your bump can reduce hip and back strain.
This is not a reason for anxiety about sleep. The absolute risk of late stillbirth is low. But given that going-to-sleep position is a modifiable factor, the evidence is strong enough to support a simple change — and the intervention costs nothing.
