The left side is the best side to sleep on during pregnancy, particularly once you reach the third trimester. Left-side sleeping allows the most blood flow to reach your baby because it keeps your weight off a major vein called the inferior vena cava, which runs along the right side of your spine and carries blood back to your heart from your lower body. That said, the right side is a perfectly good alternative, and the position you need to avoid most is flat on your back in late pregnancy.
Why the Left Side Is Recommended
As your uterus grows heavier, it can press against the inferior vena cava when you lie on your back. This compression reduces blood return to your heart, which in turn reduces how much blood reaches the placenta and your baby. Lying on the left side shifts the weight of the uterus away from this vein, maximizing circulation. Some women notice this effect directly: lying flat on their back in late pregnancy makes their heart race or leaves them feeling short of breath, lightheaded, or nauseated.
Left Side vs. Right Side
You may have heard that only the left side is safe, but the difference between left and right is smaller than many people think. An NIH-funded study that tracked sleep positions through 30 weeks of pregnancy found that adverse outcomes were no more likely among women who slept mostly on the right side compared to those who slept on the left. The researchers did note that their data only covered sleep through week 30, so the guidance for late pregnancy is less clear.
In practice, either side works well. The left side offers a slight circulatory advantage because of where the vena cava sits, but if you’re more comfortable on your right side, that’s a far better choice than lying on your back. Most people shift positions multiple times during the night anyway, so alternating between sides is completely normal.
When Sleep Position Starts to Matter
During the first and second trimesters, sleep position doesn’t appear to affect pregnancy outcomes at all. The same NIH analysis found that sleeping on your back, side, or in any other position through 30 weeks carried no increased risk of stillbirth, reduced birth size, or blood pressure complications. Even among women who spent more than half the night on their backs, outcomes were the same as those who rarely slept on their backs.
The concern kicks in during the third trimester, roughly after 28 weeks, when the uterus is heavy enough to compress blood vessels meaningfully. A large meta-analysis published in The Lancet found that going to sleep on your back after 28 weeks was associated with a 2.6-fold increase in the odds of late stillbirth compared to going to sleep on the left side. That’s a significant relative increase, though the absolute risk of late stillbirth remains low. The key takeaway: make side sleeping a habit by the start of your third trimester.
What If You Wake Up on Your Back?
This is one of the most common worries, and the answer is reassuring. Simply roll onto your side and go back to sleep. Cleveland Clinic physicians note that short periods on your back, even an hour or two, are unlikely to cause lasting harm. Your body often gives you a signal if the position is a problem: you might feel your heart beating faster, have trouble breathing, or feel dizzy. Those sensations are your cue to shift positions, and they tend to wake you up before any real issue develops.
The research on back sleeping and stillbirth risk specifically looked at the position women fell asleep in, not every position they passed through during the night. You can’t consciously control what happens while you’re asleep, so the practical advice is to start the night on your side. If you wake up on your back at 3 a.m., that’s not a reason to panic.
Pillow Strategies That Help
Side sleeping can feel awkward as your belly grows, especially if you were a back or stomach sleeper before pregnancy. A few pillow adjustments make a big difference:
- Between your knees: Keep your knees slightly bent with a pillow between them. This takes pressure off your hips and lower back by keeping your spine aligned.
- Under your belly: Tuck a pillow or wedge beneath your abdomen for support. As your belly gets heavier, this lift prevents it from pulling you forward and straining your back.
- Behind your back: Place a firm pillow along your back to keep yourself from rolling over during the night. This is especially helpful if you’re a habitual back sleeper trying to break the habit.
Full-length pregnancy pillows wrap around your body and serve all three functions at once. They’re not necessary, but many people find them easier than arranging multiple standard pillows every night. A regular body pillow works nearly as well if you pair it with a small wedge under your belly.
Stomach Sleeping During Pregnancy
Sleeping on your stomach is fine in early pregnancy when the uterus is still small and tucked behind the pelvic bone. Most women naturally stop stomach sleeping somewhere in the second trimester simply because it becomes uncomfortable. There’s no specific week when it becomes dangerous. Your body will tell you when it no longer works. Once your belly is large enough that stomach sleeping feels like balancing on a basketball, it’s time to switch to your side.