Lying on your back during pregnancy is generally fine in the first half, but after about 20 weeks, it’s best to avoid spending long stretches flat on your back. By that point, your uterus is heavy enough to compress major blood vessels, which can reduce blood flow to both you and your baby. The concern grows as pregnancy progresses, peaking in the third trimester.
Why Back Lying Becomes a Problem
Your body has two major blood vessels running along your spine: the aorta, which sends blood out to your body, and the inferior vena cava, which returns blood from your lower body back to your heart. When you lie flat on your back in later pregnancy, the weight of your uterus presses down on both of these vessels. The inferior vena cava is especially vulnerable because it sits behind the uterus and carries a large volume of blood.
When that vessel gets compressed, less blood returns to your heart, which means your heart pumps out less with each beat. This drop in circulation can lower your blood pressure significantly. In clinical terms, it’s sometimes called supine hypotensive syndrome, defined as a 30% or greater drop in systolic blood pressure compared to lying on your side. The effects range from mild and barely noticeable to, in rare cases, serious cardiovascular compromise.
What It Feels Like
Many pregnant people who lie flat on their back in the third trimester will feel something “off” within a few minutes. Common symptoms include dizziness, lightheadedness, nausea, and a sensation of breathlessness. Some people notice their heart racing as it tries to compensate for the reduced blood flow. Others feel perfectly fine, which is why this can be tricky. The absence of symptoms doesn’t necessarily mean blood flow is unaffected.
The good news is that your body often acts as its own alarm system. That wave of discomfort is a signal to roll onto your side, and doing so quickly restores normal blood flow.
The Risk to Your Baby
When blood flow through the inferior vena cava drops, less oxygenated blood reaches the placenta. Several large studies have examined whether habitually falling asleep on your back in late pregnancy raises the risk of stillbirth. A UK case-control study found that going to sleep in the supine position was associated with a 2.3 times higher risk of late stillbirth compared to falling asleep on the left side. A New Zealand study found an even stronger association, with a 3.7 times higher risk overall. For pregnancies at term (37 weeks or later), the New Zealand data showed the risk was roughly 10 times higher.
These numbers sound alarming, but context matters. Late stillbirth is rare to begin with, affecting a small fraction of pregnancies. The population attributable risk in these studies ranged from about 3.7% to 9.4%, meaning that if every pregnant person avoided falling asleep on their back, somewhere between 4 and 9 out of every 100 late stillbirths could theoretically be prevented. It’s a meaningful and modifiable risk factor, but it’s not a guarantee of harm.
When to Start Sleeping on Your Side
Most guidance points to 20 weeks as the turning point. Before that, your uterus simply isn’t large or heavy enough to compress those blood vessels in a meaningful way. Sleep however you’re comfortable in the first trimester and early second trimester.
After 20 weeks, side sleeping becomes the standard recommendation. Left side is often cited as ideal because it keeps the uterus off the inferior vena cava most effectively, but right side sleeping is also fine. The key is avoiding prolonged time flat on your back, not achieving one perfect position all night long.
What If You Wake Up on Your Back
This is probably the most common worry, and it’s worth putting it in perspective. You cannot consciously control your position while you’re asleep. Most people shift positions dozens of times per night, and waking up on your back at some point is practically inevitable.
If you wake up and realize you’re on your back, simply roll to your side. That’s it. A brief period on your back is not the same as spending the entire night in that position. The research on stillbirth risk specifically looked at the position people fell asleep in and stayed in, not momentary shifts during the night. Your body is also likely to wake you up or prompt you to move if blood flow drops enough to cause symptoms.
Practical Tips for Staying on Your Side
A pillow between your knees helps keep your hips aligned and makes side sleeping more comfortable. Many people also find that placing a firm pillow or rolled blanket behind their back prevents them from rolling fully onto their back during the night. Pregnancy pillows that wrap around your body serve the same purpose.
If you find side sleeping uncomfortable due to hip or shoulder pain, a mattress topper can help. You can also try a slight wedge or pillow under one hip so you’re tilted to the side rather than perfectly flat. Even a slight tilt off your back is enough to reduce pressure on the inferior vena cava. You don’t need to maintain a strict 90-degree side position all night.
For people who have spent their whole lives as back sleepers, the adjustment can feel frustrating. Starting to practice side sleeping before the 20-week mark, while it’s still optional, can make the transition easier when it matters more.