The left side is the most recommended sleeping position during pregnancy, but either side is safe. What matters most, especially after 28 weeks, is avoiding sleeping flat on your back for extended periods. Before that point, sleep position doesn’t appear to affect pregnancy outcomes at all.
Why Side Sleeping Matters
As your uterus grows, it becomes heavy enough to compress the inferior vena cava, the large vein that returns blood from your lower body to your heart. This compression happens when you lie flat on your back, and it reduces the amount of blood your heart can pump out with each beat. Less blood flow means less oxygen reaching the placenta and your baby.
The left side has long been considered ideal because it shifts the uterus away from this vein most effectively. For over 60 years, placing pregnant women on their left side has been standard practice in labor and delivery rooms to improve blood flow. That said, the right side also keeps pressure off the vena cava and is a perfectly reasonable alternative. An NIH-funded study of more than 8,700 women found no difference in adverse outcomes between those who slept mostly on the left side, the right side, or their back through 30 weeks of pregnancy.
When to Start Sleeping on Your Side
Current evidence points to 28 weeks as the key transition point. Before that, your uterus isn’t large enough to cause significant compression, and studies show sleep position in early and mid-pregnancy doesn’t affect the risk of stillbirth, reduced birth size, or blood pressure complications. Starting at 28 weeks, the guidance shifts: go to sleep on your side rather than your back.
The emphasis is on the position you fall asleep in. Research on stillbirth risk specifically looked at the position women went to sleep in, not every position they passed through during the night. Women who went to sleep on their backs after 28 weeks had roughly 2.6 times the odds of late stillbirth compared to those who fell asleep on their left side. That sounds alarming, but keep in mind that late stillbirth is rare to begin with, so the absolute risk remains small.
What Happens If You Wake Up on Your Back
This is one of the biggest sources of anxiety for pregnant people, and the reassuring answer is that brief periods on your back are not dangerous. Even an hour or two on your back is unlikely to harm your baby. Your body has built-in warning signals: if lying on your back is restricting blood flow, you’ll typically feel dizzy, nauseated, sweaty, or notice your heart beating faster. These symptoms usually show up within 3 to 10 minutes and resolve as soon as you shift position. In other words, your own discomfort will wake you up or prompt you to move before any real harm occurs.
If you tend to roll onto your back in your sleep, you don’t need to stay perfectly on your side all night. Even a slight incline helps. Propping yourself at a 20- to 30-degree angle with a pillow behind your back relieves pressure on the vena cava. You’re not aiming for perfection. You’re aiming to avoid spending hours completely flat.
How to Stay Comfortable on Your Side
Side sleeping can get uncomfortable fast when you’re carrying extra weight in front. The key is keeping your spine neutral so your hips, pelvis, and lower back aren’t torqued. Place a pillow (or several) between your knees, thighs, and feet so that your upper leg sits level with your pelvis and mirrors the position of your bottom leg. This prevents your top knee from pulling your spine out of alignment.
Shoulder and arm pain are common complaints, especially if you’re spending more time on one side than you’re used to. Placing pillows behind your back and hips lets you roll back slightly onto them, taking weight off your shoulder without ending up flat. A full-length pregnancy pillow can do this in one piece, but a few regular pillows work just as well.
Switching between your left and right side throughout the night is completely fine and actually helps prevent soreness from building up on one side. You don’t need to stay locked on your left all night.
Left vs. Right: Does It Really Matter?
The left side has a slight physiological edge because the inferior vena cava runs along the right side of the spine, so left-side sleeping moves the uterus furthest from it. Studies on cardiac output confirm that the left lateral position produces the best blood flow numbers compared to supine or even right-side lying. But the practical difference between left and right is small. The NIH study that tracked sleep positions with wearable devices found no increase in complications among women who didn’t favor the left side. The meaningful distinction is side versus back, not left versus right.
If you’re more comfortable on your right side, stay there. If you naturally prefer the left, even better. The goal is to fall asleep on either side after 28 weeks, not to stress over which one.