The left side is the best sleeping position during pregnancy, but either side is safe. The key recommendation from major health organizations is to avoid falling asleep on your back after 28 weeks of pregnancy. If you’re earlier than that, sleep position doesn’t appear to matter much at all.
Why the Left Side Is Preferred
Your left side allows maximum blood flow to the baby and improves kidney function. The reason comes down to anatomy: a major blood vessel called the inferior vena cava runs along the right side of your spine, carrying blood from your lower body back to your heart. When you lie on your left side, the weight of your uterus falls away from this vessel, keeping circulation flowing freely.
Better blood return means your heart pumps more efficiently, your kidneys filter waste more effectively, and your baby receives more oxygen and nutrients through the placenta. It can also help reduce swelling in your legs and feet, since blood isn’t pooling in your lower extremities.
Right Side Sleeping Is Fine Too
If you’ve heard that sleeping on your right side is risky, you can relax. A review by the UK’s National Institute for Health and Care Excellence found no increased likelihood of stillbirth associated with going to sleep on the right side compared to the left. The evidence was clear enough that the guideline committee specifically called this out, noting that concerns about right-side sleeping are not supported by the data.
So while the left side is theoretically optimal for blood flow, either side is a safe choice. The real concern is back sleeping in the third trimester.
Why Back Sleeping Becomes a Problem
When you lie flat on your back in late pregnancy, the full weight of your uterus presses down onto the inferior vena cava and the aorta against your spine. This can drastically reduce blood flow returning from your lower body to your heart, a condition sometimes called supine hypotensive syndrome. Symptoms include dizziness, nausea, sweating, pallor, and a drop in blood pressure. Turning onto your side resolves it almost immediately.
A large New Zealand study found that going to sleep on your back was associated with a 3.7-fold increase in the risk of late stillbirth (after 28 weeks), independent of other common risk factors. The researchers estimated that a public health campaign encouraging side sleeping could reduce late stillbirths by roughly 9%.
When Sleep Position Starts to Matter
In early and mid-pregnancy, your sleep position doesn’t appear to affect outcomes. An NIH-funded analysis tracked sleep positions through 30 weeks and found no increased risk of stillbirth, reduced fetal growth, or high blood pressure disorders among women who slept on their backs or right sides compared to those who slept on their left.
The 28-week mark is the threshold most guidelines use. After that point, the uterus is heavy enough to meaningfully compress blood vessels when you’re on your back. Before 28 weeks, sleep however you’re comfortable.
What If You Wake Up on Your Back
This is one of the most common worries, and it’s worth putting in perspective. The research focuses on the position you fall asleep in, not what happens during the night. You can’t control how your body shifts while you’re unconscious. If you wake up on your back, simply roll to your side and go back to sleep. Your body often gives you signals before any real harm occurs: the dizziness, nausea, or discomfort from reduced blood flow typically wakes you up and prompts you to move.
The goal is to start the night on your side. Don’t lose sleep over the possibility of rolling onto your back, because the anxiety itself is counterproductive when rest is what you need most.
Pillow Setups That Help
Staying on your side all night gets harder as your belly grows. Pillows are your best tool for staying comfortable and staying put.
- Between your knees and thighs: Place a pillow (or a full-length body pillow) so that your upper leg is level with your pelvis and mirrors your bottom leg. This keeps your hips aligned and takes pressure off your lower back.
- Under your belly: A small rolled towel or thin pillow tucked beneath your abdomen supports the weight of your uterus and prevents it from pulling you forward.
- Behind your back: A firm pillow wedged behind your back, hips, and legs makes it harder to roll onto your back during the night. It also lets you lean slightly backward without going fully supine, which can relieve pressure on the shoulder you’re lying on.
- Between your ribs and hips: If you feel a gap at your waist when lying on your side, a rolled towel here supports your spine in a neutral position.
- Under your neck: A small rolled washcloth under the curve of your neck, in addition to your regular pillow, can prevent neck stiffness from hours in one position.
Full-length pregnancy pillows (C-shaped or U-shaped) combine several of these functions into one piece, wrapping around your body to support your back, belly, and knees simultaneously. They’re not necessary, but many women find them easier than arranging four or five separate pillows each night. Experiment with what feels right. The best setup is the one that lets you fall asleep on your side and actually stay comfortable enough to get decent rest.