Most experts recommend stopping back sleeping around 28 weeks of pregnancy, which is the start of the third trimester. Before that point, sleeping position does not appear to affect pregnancy outcomes. After 28 weeks, falling asleep on your back is linked to a meaningful increase in stillbirth risk and reduced blood flow to your baby.
Why 28 Weeks Is the Threshold
A UK clinical guideline reviewed the available evidence and concluded that women should try to avoid going to sleep on their back after 28 weeks. The reasoning is straightforward: by the third trimester, your uterus is heavy enough to compress the large vein that carries blood back to your heart when you lie flat. This compression reduces blood return to your heart, lowers your cardiac output, and cuts the blood supply reaching your baby through the placenta.
Earlier in pregnancy, the uterus simply isn’t large enough to cause this problem. A large NIH-funded study found that sleeping position during early and mid-pregnancy had no effect on the risk of complications. Women who slept on their back or right side before the third trimester had the same outcomes as those who slept on their left side. So if you’re in your first or second trimester, there’s no need to worry about your sleep position yet.
The Risks of Back Sleeping After 28 Weeks
A New Zealand case-control study found that going to sleep on the back in late pregnancy was associated with a 3.7-fold increase in late stillbirth risk, independent of other common risk factors. The researchers estimated that if all women avoided falling asleep on their backs in the third trimester, roughly 9% of late stillbirths could be prevented.
The effects extend beyond stillbirth. Research published in JAMA Network Open found that women at 28 weeks or beyond who regularly fell asleep on their back gave birth to babies weighing an average of 144 grams less than babies born to side sleepers (about 3,410 g versus 3,554 g). Back sleepers also had a threefold increase in the odds of having a small-for-gestational-age baby. Reduced blood flow from back sleeping doesn’t just pose an acute risk; it can affect how well your baby grows over the final weeks of pregnancy.
Left Side, Right Side, or Either?
You may have heard that left-side sleeping is the only safe option, but the evidence is more nuanced than that. The NIH-funded study that tracked sleep position and outcomes found no difference in complication rates between women who slept on their left side and those who slept on their right. The key distinction is side sleeping versus back sleeping, not left versus right.
That said, some earlier studies did link right-side sleeping with slightly higher risks, and left-side sleeping has a theoretical advantage: it keeps the weight of the uterus off both the large vein returning blood to your heart and the aorta. In practice, either side is a reasonable choice. Most women naturally shift between sides throughout the night, and that’s perfectly fine.
What If You Wake Up on Your Back?
This is one of the most common worries, and it deserves a clear answer: the research focuses on the position you fall asleep in, not every position you pass through during the night. The New Zealand stillbirth study specifically measured “going-to-sleep position,” meaning the position you settle into when you drift off. You can’t consciously control what happens while you’re asleep, and brief periods on your back are not the same as spending the entire night supine.
If you wake up on your back, simply roll to your side and go back to sleep. Many women find that as pregnancy progresses, lying on the back becomes uncomfortable on its own. You may feel lightheaded, short of breath, or nauseous, which are signs of reduced blood flow. Your body often gives you a natural signal to move before any harm is done.
Practical Ways to Stay on Your Side
If you’re not naturally a side sleeper, a few simple strategies can help you stay off your back through the third trimester.
- Full-body pregnancy pillows: C-shaped or U-shaped pillows wrap around your body and support your head, belly, back, and knees simultaneously. The back support acts as a physical barrier that makes rolling over less likely. Popular options include long C-shaped pillows that can be twisted into different configurations and U-shaped pillows that cradle you from both sides.
- A pillow behind your back: If you don’t want a full-body pillow, tucking a firm regular pillow or rolled blanket behind your back creates enough of a wedge to keep you from rolling flat.
- A pillow between your knees: This keeps your hips aligned and makes side sleeping more comfortable, so you’re less tempted to shift onto your back to relieve pressure.
- A slight tilt: You don’t need to be perfectly on your side. Even a slight angle, with your back propped up 15 to 30 degrees rather than lying completely flat, helps keep the uterus from compressing the major blood vessels.
The transition takes some getting used to, especially if you’ve been a lifelong back sleeper. Starting to practice side sleeping a few weeks before the 28-week mark can make the shift feel more natural by the time it matters most. Most women find that within a week or two, side sleeping with the right pillow support becomes comfortable enough that they stop thinking about it.