Either side is fine. The American College of Obstetricians and Gynecologists says you can sleep on your right or left side during pregnancy, and the best large-scale research confirms that outcomes are essentially the same for both. Left-side sleeping has long been the default recommendation, but the evidence has shifted, and the current advice is simpler: sleep on whichever side is comfortable.
Why Side Sleeping Matters in Pregnancy
The real concern isn’t left versus right. It’s about avoiding prolonged back sleeping in the later months. When you lie flat on your back, the weight of your uterus presses on the inferior vena cava, a major vein that returns blood from your lower body to your heart. That compression can reduce blood flow to the placenta and make you feel lightheaded, short of breath, or like your heart is racing.
A meta-analysis published in The Lancet’s eClinicalMedicine, pooling data from five studies with over 3,000 participants, found that going to sleep on your back in late pregnancy was associated with 2.6 times higher odds of late stillbirth compared to going to sleep on the left side. Critically, the same analysis found that sleeping on the right side carried virtually identical odds to the left (a difference so small it was statistically meaningless). The takeaway: side sleeping is protective, and it doesn’t matter which side you choose.
Where the “Always Sleep on Your Left” Advice Came From
The left-side recommendation made anatomical sense on paper. The inferior vena cava runs slightly to the right of your spine, so lying on your left theoretically takes more pressure off it. Left-side sleeping also appears to mildly improve kidney function and blood flow to the placenta in short-term measurements.
But when researchers at the National Institutes of Health looked at actual pregnancy outcomes in a large prospective study, they found no association between non-left sleeping positions and any adverse outcome. The editorial accompanying that study in Obstetrics & Gynecology was blunt: pregnant women should be told that the best evidence does not support a link between right-side sleeping and complications, and they should feel comfortable sleeping in whatever position works for them.
One Exception: Acid Reflux
If heartburn is keeping you up, left-side sleeping does have a specific advantage. Multiple studies have found that lying on your left side reduces the number of reflux episodes and limits how much stomach acid reaches the esophagus. The anatomy works in your favor here: your stomach curves in a way that keeps acid pooled away from the opening to your esophagus when you’re on your left. Since reflux is extremely common in the third trimester, this is one situation where choosing the left side can make a noticeable difference in comfort.
What If You Wake Up on Your Back
It happens to almost everyone. You fall asleep on your side and wake up staring at the ceiling. This is not a cause for panic. Cleveland Clinic physicians note that short periods on your back, even an hour or two, are unlikely to harm your baby. Your body typically sends clear signals when blood flow is being compromised: you’ll feel uncomfortable, a bit breathless, or notice your heart beating faster. Those sensations are your cue to roll back onto your side.
A simple trick is to place a pillow between your back and the mattress. Even if you roll during the night, you’ll end up on a slight incline rather than flat on your back. As long as you’re angled 20 to 30 degrees, the pressure on the vena cava is substantially relieved.
How to Stay Comfortable on Your Side
Side sleeping gets harder as your belly grows. Hip pain, shoulder aches, and lower back strain are common complaints, especially in the third trimester. Strategic pillow placement makes a big difference.
- Between your knees and thighs: Place a pillow (or pillows) so your upper leg is level with your pelvis and mirrors the position of your bottom leg. This keeps your hips aligned and takes pressure off your lower back and pelvic joints.
- 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 along your back and hips lets you lean slightly backward, which relieves pressure on your bottom shoulder and arm. It also serves as a barrier against rolling fully onto your back.
Full-length pregnancy pillows combine all three of these functions into one piece, which some people find easier to manage than juggling multiple pillows throughout the night. The shape you choose (C, U, or wedge) comes down to personal preference and how much bed space you have.
When Sleep Position Starts to Matter
In the first trimester, your uterus is still small enough that sleeping position has no meaningful effect on blood flow. Sleep however you’re comfortable, including on your stomach if that still feels fine. Most people naturally stop stomach sleeping by the second trimester simply because it becomes uncomfortable.
The research on back sleeping and stillbirth risk focuses on late pregnancy, generally after 28 weeks. That’s the point at which the uterus is heavy enough to compress the vena cava when you’re supine. Shifting to side sleeping as a habit sometime in the second or early third trimester gives you time to adjust before it becomes more important. But even then, the goal is your going-to-sleep position. You can’t control what happens while you’re unconscious, and the data is based on the position women fall asleep in, not every position they pass through during the night.