Side sleeping is the safest position during pregnancy, especially from 28 weeks onward. Either side works. Left side sleeping has long been considered ideal, but research from the National Institutes of Health found that sleeping on your back or right side during early and mid pregnancy does not increase the risk of complications. The real shift happens in the third trimester, when the weight of the uterus becomes significant enough to compress major blood vessels if you lie flat on your back.
Why Side Sleeping Matters After 28 Weeks
As your uterus grows, it gets heavy enough to press against two critical blood vessels that run along your spine: the main artery supplying your lower body and the large vein that returns blood from your legs back to your heart. When you lie flat on your back in the second half of pregnancy, this compression can reduce the blood returning to your heart by 25% to 30%, dropping your cardiac output and potentially reducing blood flow to the placenta.
A large meta-analysis published in eClinicalMedicine found that women who fell asleep on their backs after 28 weeks had roughly 2.6 times the odds of late stillbirth compared to those who fell asleep on their left side. The researchers estimated that if every woman in the third trimester simply settled to sleep on her side, late stillbirth could be reduced by about 6%. The takeaway is straightforward: from 28 weeks on, start the night on either side.
Left Side vs. Right Side
You may have heard that the left side is the only correct option. That advice is outdated. The NIH-funded study specifically addressed this, finding that right-side sleeping in early and mid pregnancy carried no additional risk. The left side does offer a slight theoretical advantage because it keeps the uterus off the large vein, which sits slightly to the right of your spine, and may optimize blood flow to the kidneys. But both sides are safe. If you’re more comfortable on your right, stay there.
What If You Wake Up on Your Back?
This is one of the most common worries, and the answer is reassuring. Brief periods on your back, even an hour or two, are unlikely to cause harm. Your body has built-in warning signals: if back sleeping starts to compromise blood flow, you’ll typically feel short of breath, dizzy, or notice your heart beating faster. That discomfort prompts you to roll over before any real problem develops.
If you wake up on your back, simply shift to your side and go back to sleep. You don’t need to panic. You also don’t need to be perfectly flat on your side. Even a 20- to 30-degree angle is enough to relieve pressure on the blood vessels. Tucking a pillow behind your back can keep you from rolling fully flat during the night.
How to Get Comfortable on Your Side
Pillows are the most effective tool for making side sleeping sustainable night after night. The goal is to keep your spine neutral and your hips aligned so nothing pulls or sags. Here’s how to set up:
- Between your knees and thighs: Place a pillow (or a full-length body pillow) so your upper leg is level with your pelvis and mirrors the position of your bottom leg. This prevents your top knee from dropping forward and twisting your lower back.
- Under your belly: A small rolled towel or thin pillow tucked beneath your abdomen supports the weight of your uterus and keeps it from pulling on your back muscles.
- Along your spine: If you feel unstable, a rolled towel or pillow between your ribs and hips can provide extra support and keep you from rolling backward.
- Behind your back: A firm pillow wedged between your back and the mattress acts as a physical barrier against rolling onto your back while you sleep.
Full-length pregnancy pillows (C-shaped or U-shaped) combine several of these functions into one product, which some women find simpler than arranging multiple pillows. Others prefer the flexibility of individual cushions they can adjust throughout the night.
Dealing With Heartburn at Night
Acid reflux worsens when you lie flat because gravity no longer keeps stomach acid down. In the third trimester, the uterus pushes the stomach upward, making the problem even worse. Elevating your upper body with a wedge pillow or raising the head of your bed by 4 to 6 inches can make a noticeable difference. This works in combination with side sleeping: you can lie on your side while still being slightly propped up.
Sleep Challenges by Trimester
First Trimester
Rising levels of estrogen and progesterone disrupt your breathing regularity and sleep cycles, often causing fragmented sleep and daytime exhaustion even though your belly is still small. Nausea can also wake you. Position doesn’t matter much yet, so sleep however feels comfortable. This is a good time to start building the side-sleeping habit if you’re naturally a back or stomach sleeper.
Second Trimester
Many women get a reprieve during the middle months. Nausea fades, and the belly isn’t yet large enough to cause major discomfort. You may start noticing that lying flat on your back feels less comfortable as your uterus grows. A pillow between your knees can ease early hip pressure.
Third Trimester
This is when sleep becomes genuinely difficult. The weight of the baby presses on your joints, lower back, and bladder. Frequent urination, back pain, and difficulty finding a comfortable position are all common. Leg cramps and restless legs also peak during this stage.
Managing Leg Cramps and Restless Legs
Nocturnal leg cramps are one of the most disruptive sleep problems in late pregnancy. Stretching your calves before bed can help prevent them. A simple wall stretch works well: stand at arm’s length from a wall, step one foot behind the other, and slowly lean forward while keeping the back heel flat on the floor. Hold for about 30 seconds, then switch sides.
Regular physical activity during the day also reduces the frequency of cramps. Research suggests that lower blood calcium levels during pregnancy may contribute to cramping, and getting 1,000 milligrams of calcium daily through food or supplements is recommended. Magnesium supplements show some promise for prevention as well, though the evidence is mixed. Magnesium-rich foods like nuts, seeds, beans, whole grains, and dried fruits are worth adding to your diet regardless. Wearing supportive shoes during the day, particularly ones with a firm heel counter, can reduce leg strain that contributes to nighttime cramps.
If a cramp strikes in the middle of the night, flex your foot by pulling your toes toward your shin to stretch the calf muscle. The cramp typically releases within a minute.