Most guidelines recommend stopping back sleeping around 28 weeks of pregnancy, which is the start of the third trimester. Before that point, sleeping on your back or side does not appear to increase the risk of complications. After 28 weeks, the weight of the uterus becomes significant enough to compress major blood vessels when you lie flat, which can reduce blood flow to both you and your baby.
Why 28 Weeks Is the Turning Point
The uterus sits at the same level as the lower lumbar spine, right in front of the body’s largest vein (the inferior vena cava) and the aorta. When you’re upright or on your side, gravity keeps the uterus from pressing on these vessels. But when you lie flat on your back, the full weight of the uterus settles directly onto them, squeezing the blood flow returning from your lower body to your heart.
This matters more as pregnancy progresses because the blood flowing through the uterus increases dramatically, from about 60 milliliters per minute before pregnancy to roughly 600 milliliters per minute at full term. That tenfold increase means even a mild drop in returning blood flow can have outsized effects on both your circulation and your baby’s oxygen supply. Before 28 weeks, the uterus simply isn’t heavy enough to cause this level of compression in most women.
An NIH-funded study confirmed this timeline: sleeping on the back or side through 30 weeks did not increase the risk of stillbirth, reduced birth size, or blood pressure complications. The researchers specifically noted their findings did not extend beyond 30 weeks, reinforcing that late pregnancy is when position starts to matter.
What Back Sleeping Does to Your Baby
When the vena cava is compressed, less blood reaches the placenta. Research shows that babies respond to this by redistributing their own blood circulation, increasing flow to the brain, a sign of mild oxygen stress. They also become quieter and less active, which is another adaptation to reduced oxygen.
A large meta-analysis found that women who regularly fell asleep on their back after 28 weeks gave birth to babies weighing an average of 144 grams less than those who slept in other positions. That’s roughly a third of a pound, and it represented about a 10% reduction in birth weight percentile, independent of other factors that affect size. The researchers described this as clinically significant and, importantly, modifiable.
The stillbirth data is more striking. Women who went to sleep on their back had roughly 2.6 times the odds of late stillbirth compared to those who fell asleep on their left side. Researchers estimated that if all women avoided the supine position when going to sleep, late stillbirth rates could be reduced by about 6%. The UK’s National Institute for Health and Care Excellence reviewed this evidence and concluded it was strong enough to formally advise women to avoid falling asleep on their back after 28 weeks.
Left Side vs. Right Side
You may have heard that sleeping on your left side is best. The reasoning is that the vena cava runs slightly to the right of the spine, so left-side sleeping theoretically keeps the most pressure off of it. In practice, though, the NIH-funded study found no difference in complications between women who slept mostly on their left side and those who slept on their right through 30 weeks. Either side is a good choice. The key distinction in late pregnancy is side sleeping versus back sleeping, not which side you pick.
If You Wake Up on Your Back
This is one of the most common worries, and the reassuring answer is that brief periods on your back are not likely to cause harm. Cleveland Clinic specialists note that even an hour or two on your back probably poses no danger to your baby. The concern is with habitually falling asleep supine night after night, not occasional rolling.
Your body also has a built-in warning system. When the vena cava is compressed enough to matter, you’ll often feel short of breath, lightheaded, or notice your heart beating faster. These symptoms are your cue to roll onto your side, and they resolve quickly once you do. Many women find they can’t stay on their back comfortably in late pregnancy even if they try, precisely because of these signals.
Practical Ways to Stay on Your Side
If you’ve always been a back sleeper, retraining yourself takes some strategy. The most effective approach is using pillows to physically prevent you from rolling flat.
- Behind your back: Place a firm pillow or wedge behind your back and hips. This creates a barrier that stops you from rolling fully supine. Even if you lean back against it, you’ll stay at a tilt rather than lying flat, which is enough to keep pressure off the vena cava.
- Between your knees: A pillow between your knees, thighs, and feet keeps your upper leg level with your pelvis and your spine in a neutral position. This makes side sleeping more comfortable and sustainable through the night.
- Under your belly: A small rolled towel or pillow tucked under your abdomen supports the weight of the uterus so it doesn’t pull you forward uncomfortably.
- Along your torso: A full-length pregnancy pillow combines several of these functions, giving you something to drape your arm and leg over while keeping your back supported.
If you do end up on your back despite these precautions, a wedge pillow underneath your upper body can keep you from lying completely flat. Even a slight incline or tilt to one side reduces the compression on blood vessels significantly compared to lying fully supine on a flat surface. The goal isn’t perfection. It’s making side sleeping your default position when you drift off, and giving yourself physical cues that nudge you back if you shift during the night.