Side sleeping is the safest position during pregnancy, and the left side has a slight edge, but both sides are fine. The key recommendation from medical guidelines is to avoid falling asleep on your back after 28 weeks of pregnancy, when the growing uterus is heavy enough to compress major blood vessels. Before that point, sleep position doesn’t appear to affect the risk of complications.
Why Left Side Sleeping Gets Recommended
The left-side preference comes down to anatomy. A large blood vessel called the inferior vena cava runs along the right side of your spine, carrying blood back to your heart from your lower body. As the uterus grows, its weight can press against this vessel when you lie flat on your back, reducing blood return to the heart and, in turn, blood flow to the placenta. Sleeping on your left side shifts the uterus away from that vessel entirely, giving blood the clearest path back to the heart.
This is why left-side sleeping has been the traditional advice from maternity care providers for decades. It maximizes circulation to the placenta and can help your kidneys work more efficiently at filtering fluid, which may reduce swelling in your legs and feet.
Right Side Sleeping Is Safe Too
Despite older studies linking right-side sleeping with slightly higher risks of stillbirth and low birth weight, a large study funded by the National Institutes of Health found no increased risk of complications for women who slept mostly on the right side compared to the left. The researchers tracked adverse outcomes in roughly 1,900 pregnancies and found they were no more common among right-side or back sleepers during early and mid pregnancy.
The practical takeaway: either side works. If the left side feels uncomfortable or you naturally roll to the right during the night, there’s no reason to force yourself back. What matters most is avoiding prolonged back sleeping in the third trimester.
Back Sleeping After 28 Weeks
A major 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. The population-level impact was estimated at about 5.8%, meaning roughly 1 in 17 late stillbirths could potentially be linked to supine sleep position. UK clinical guidelines now advise women to try to avoid going to sleep on their back after 28 weeks based on this evidence.
The mechanism is straightforward. When you lie flat, the full weight of the uterus, baby, placenta, and amniotic fluid presses down on the inferior vena cava and the aorta. This can lower your blood pressure and reduce oxygen delivery to the baby. Some women feel this happening: dizziness, nausea, a racing heartbeat, or feeling clammy and pale. These symptoms resolve quickly once you roll onto your side. In rare cases, severe compression can cause fainting.
Your body often signals the problem before any harm is done. If you feel short of breath or your heart starts pounding while lying on your back, that’s your cue to shift to your side.
What If You Wake Up on Your Back
This is one of the most common worries, and the answer is reassuring. Waking up on your back occasionally is not dangerous. The concern is about spending the entire night in a supine position, not brief periods. Short stretches of an hour or two on your back are unlikely to cause harm. The studies that found increased stillbirth risk looked at women who reported sleeping on their back for the whole night, not those who shifted positions naturally.
Your body has built-in warning signs. If the compression becomes significant, you’ll typically feel uncomfortable enough to move, even in your sleep. If you wake up on your back, simply roll to whichever side feels comfortable and go back to sleep. There’s no need to panic or lose sleep over it, which would be counterproductive since sleep quality matters during pregnancy too.
When Sleep Position Starts to Matter
In early and mid pregnancy, your sleep position doesn’t appear to affect outcomes. The uterus is still small enough that it isn’t compressing blood vessels significantly. By around 20 weeks (five months), the uterus has grown large enough to start putting pressure on the vena cava and aorta when you’re on your back. The clinical guidelines focus on 28 weeks as the threshold because that’s when the research shows a measurable increase in stillbirth risk from supine sleeping.
So for the first two trimesters, sleep however you’re comfortable. As you move into the third trimester, make a habit of falling asleep on your side. The position you fall asleep in is the one you spend the most time in, so that’s what matters most.
How to Stay Comfortable on Your Side
Side sleeping gets harder as pregnancy progresses. Hip pain, lower back pressure, and the sheer size of your belly can make it difficult to stay in one position for long. The right pillow setup makes a significant difference.
Place a pillow (or several) between your knees, thighs, and feet so that your upper leg sits level with your pelvis and mirrors the angle of your bottom leg. This keeps your hips aligned and takes pressure off your lower back. A small rolled towel or thin pillow tucked under your belly supports the weight of your uterus so it doesn’t pull downward. If you’re getting lower back pain, try a rolled towel placed between your ribs and hips to keep your spine in a neutral position.
Full-length pregnancy pillows, C-shaped or U-shaped, essentially do all of this at once. They support your belly, go between your knees, and provide something to lean against so you’re less likely to roll onto your back. Some women also find that a firm pillow placed behind their back acts as a physical barrier against rolling over during sleep.
If one hip gets sore, switch to the other side. Alternating between left and right throughout the night is completely fine and keeps pressure from building up in one spot. The goal is falling asleep on either side, not staying perfectly still all night.