Most guidance says to stop going to sleep on your back after 28 weeks of pregnancy. Before that point, your sleep position doesn’t appear to affect your risk of complications. The concern is specific to the third trimester, when the weight of the uterus can press on major blood vessels and reduce blood flow to both you and your baby.
Why Back Sleeping Becomes a Problem
Your uterus sits at roughly the same level as your lower spine, right in front of the inferior vena cava, the large vein that carries blood from your lower body back to your heart. When you’re upright or sitting, gravity pulls the uterus forward and away from that vein. When you lie flat on your back, the uterus settles backward onto the spine, and its weight can compress the vena cava and the aorta underneath it.
This compression reduces the volume of blood returning to your heart, which in turn lowers the blood flow reaching the placenta and your baby. In early pregnancy, the uterus is too small and light for this to matter. By the third trimester, it’s heavy enough to cause real circulatory changes.
The 28-Week Cutoff
An NIH-funded study tracking sleep positions during early and mid-pregnancy found that sleeping on your back, right side, or left side made no difference in outcomes like stillbirth, low birth weight, or preeclampsia during the first two trimesters. The researchers concluded that sleep position before the third trimester does not appear to affect complication risk.
The picture changes after 28 weeks. A large New Zealand case-control study found that women who went to sleep on their backs in the third trimester had a 3.7-fold increased risk of late stillbirth, independent of other common risk factors. The association was even stronger at term: women who fell asleep on their backs after 37 weeks had roughly 10 times the risk compared to those who fell asleep on their side. A separate study found that women who reported going to sleep supine in late pregnancy had babies that weighed an average of 144 grams less than babies of women who slept on their side, a meaningful difference in growth.
Based on this body of evidence, clinical guidelines now advise trying to avoid going to sleep on your back after 28 weeks.
Left Side vs. Right Side
You may have heard that left-side sleeping is the only safe option. Earlier studies did link right-side sleeping with slightly higher risks, and many physicians still recommend the left side because it theoretically keeps the uterus off the vena cava most effectively. But the NIH-funded study found no meaningful difference in outcomes between left-side and right-side sleepers through mid-pregnancy, and more recent guidance treats any side-lying position as acceptable. The key distinction is side versus back, not left versus right.
What If You Wake Up on Your Back
This is the part that causes the most anxiety, and it shouldn’t. The position you fall asleep in is the one you spend the most time in overnight, which is why the research focuses on “going-to-sleep position.” Rolling onto your back briefly during the night is normal and not something to panic about.
Your body has a built-in warning system. If the compression is significant enough to threaten your baby’s oxygen supply, you’ll feel it first: nausea, dizziness, sweating, a racing heart, or breathlessness. These symptoms are your cue to roll onto your side, and they resolve quickly once you do. Most women never experience these symptoms at all, which means that waking up on your back occasionally is unlikely to have caused harm.
In rare, severe cases, the drop in blood pressure can cause fainting. But again, this would wake you or alert you well before it became dangerous to your baby. If you find yourself frequently waking up on your back, you can ask your partner to check on you during the night and gently nudge you onto your side.
Staying on Your Side Through the Night
Telling yourself to sleep on your side is one thing. Actually staying there for eight hours is another, especially as your belly grows and comfort becomes harder to find. A few strategies can help.
Pillows are the simplest solution. A randomized trial of 35 pregnancies observed over 469 nights found that both pregnancy-specific pillows and regular pillows were equally effective at reducing time spent on the back. Even with pillows, women still spent about an hour per night (13 to 16 percent of the night) on their backs, which reinforces that some back time is nearly unavoidable and not cause for alarm. Placing a pillow behind your back creates a physical barrier that makes it harder to roll fully supine. A pillow between your knees can also make side-lying more comfortable and reduce hip and lower back strain.
Positional therapy devices, borrowed from sleep medicine where they’re used to treat snoring and sleep apnea, are another option. Two feasibility studies involving 45 pregnant individuals across 90 nights in the third trimester found that a wearable positional device significantly reduced time spent supine. That said, a larger randomized trial of 181 participants using the same device didn’t find a significant effect on birth weight compared to a sham device, so the clinical benefit of these gadgets beyond simple pillow use remains unclear.
The Bottom Line on Timing
Before 28 weeks, sleep however you’re comfortable. After 28 weeks, make a habit of falling asleep on your side. Either side works. If you wake up on your back, simply roll over. No pillow setup or device is perfect, and the goal is to reduce total supine time, not eliminate every second of it. The position you intentionally choose when you settle in for the night is what matters most.