From 28 weeks of pregnancy onward, fall asleep on your side — left or right is fine; do not fall asleep on your back

No money spent. The hard part is changing posture if you are used to sleeping on your back

Stillbirth here means the baby dying in the womb after 28 weeks. One pool merged raw data from five studies. Four were in New Zealand, Australia, and the UK; one was an online international survey. There were 851 pregnant women with stillbirth and 2257 with a live child as controls. The method was to ask afterward how they had fallen asleep most recently. Versus falling asleep on the left, falling asleep on the back had an adjusted odds ratio for stillbirth of aOR 2.63 (95% CI 1.72–4.04, the plausible range), about 2.6 times. Falling asleep on the right aOR 1.04 (0.83–1.31), no difference from the left. The association was similar when the child was small, the pregnant person was obese, or smoked. The authors estimated that if all pregnant people after 28 weeks fell asleep on their side, stillbirths in that stage would fall by about 5.8% (3.2%–9.2%). The UK National Health Service (NHS) lists “do not fall asleep on your back after 28 weeks” among ways to lower stillbirth risk. It also says that waking up on your back is not a reason to worry — roll to the side and sleep on. Stillbirth is rare, so its odds ratio and relative risk are close.

Cronin RS, Li M, Thompson JMD, et al. (2019). An Individual Participant Data Meta-analysis of Maternal Going-to-Sleep Position, Interactions with Fetal Vulnerability, and the Risk of Late Stillbirth. EClinicalMedicine, 10, 49-57. https://doi.org/10.1016/j.eclinm.2019.03.014；Silver RM, Hunter S, Reddy UM, et al. (2019). Prospective Evaluation of Maternal Sleep Position Through 30 Weeks of Gestation and Adverse Pregnancy Outcomes. Obstetrics and Gynecology, 134(4), 667-676. https://doi.org/10.1097/AOG.0000000000003458（备注里的反方研究）；NHS. Stillbirth. https://www.nhs.uk/conditions/stillbirth/

Controversial. The figures above all come from asking afterward. People whose baby died may remember inaccurately; the authors acknowledge that. There is also no randomized trial. A US study asked 8706 first-time pregnant women about sleep position in advance, then looked at outcomes. Through 30 weeks, back sleeping or not sleeping on the left had no link to stillbirth, a small baby, or pregnancy hypertension (aOR 0.99, 0.89–1.11). But it only reached 30 weeks; sleep position in the later weeks was not recorded. Side sleeping costs nothing and has no downside, so the UK still advises it. Stillbirth itself is uncommon; in high-income countries late stillbirth is about 1.3 to 8.8 per 1000 births. The “large” benefit rating follows the odds ratio of 2.63 mechanically, with the reasoning in the verification record. The benefit falls on the child