Sleeping on your left side during pregnancy is traditionally recommended because it keeps the weight of your growing uterus off a major blood vessel called the inferior vena cava, which runs along the right side of your spine. This allows the best possible blood flow to your baby and your kidneys. That said, the science on this topic has evolved, and the current evidence is more reassuring than many pregnant people expect.
What Happens When You Sleep on Your Back
The real issue isn’t left versus right. It’s side versus back. After about 20 weeks, the uterus is heavy enough to compress both the inferior vena cava and the aorta against your spine when you lie flat. This compression reduces the amount of blood returning to your heart, which can drop your cardiac output by 25% to 30%. In about 15% of women at term, this triggers what’s called supine hypotensive syndrome: a sudden drop in blood pressure of 15 to 30 mmHg, sometimes with dizziness, nausea, or a racing heart. In severe cases, cardiac output can fall by 30% to 40%.
When less blood circulates back to your heart, less oxygenated blood reaches the placenta. Your baby depends entirely on that blood flow for oxygen and nutrients, so anything that reduces it for extended periods is worth avoiding.
Why the Left Side Gets Special Mention
The inferior vena cava sits slightly to the right of your spine. Lying on your left side shifts the uterus away from this vein completely, giving blood the clearest path back to your heart. Left-side sleeping also improves kidney function, which helps your body filter waste more efficiently and can reduce swelling in your feet and ankles. More blood flow to the kidneys means more fluid is processed and excreted rather than pooling in your lower legs.
This is why health sources have long called the left side the “ideal” position. It maximizes blood flow to both the baby and your kidneys in one simple position change.
Left Side vs. Right Side: What the Evidence Says
Here’s where the advice has shifted. A large meta-analysis published in The Lancet’s eClinicalMedicine found that going to sleep on your back was associated with 2.6 times higher odds of late stillbirth compared to the left side. But sleeping on the right side carried virtually the same risk as the left, with no statistically meaningful difference between the two.
An NIH-funded study also found that sleep position through 30 weeks of pregnancy didn’t affect the risk of stillbirth, reduced birth size, or blood pressure disorders, regardless of whether women slept on their back, left, or right side. The researchers noted that compression becomes more relevant in the final weeks as the uterus reaches its heaviest.
Based on this evidence, a 2025 review in the Canadian Medical Association Journal concluded that going to sleep on either side is equally safe starting at 28 weeks. The key recommendation is to avoid falling asleep on your back after that point. Before 28 weeks, sleep position does not appear to affect pregnancy outcomes at all.
When Sleep Position Starts to Matter
Current evidence points to 28 weeks as the threshold. Before that, sleep however you’re comfortable. After 28 weeks, the uterus is large and heavy enough that back sleeping can meaningfully reduce blood flow. This applies to all pregnancies, not just those considered high-risk.
The population-level impact is relatively small. The meta-analysis estimated that back sleeping accounts for roughly 5.8% of late stillbirths. That means the vast majority of women who occasionally end up on their backs will have perfectly healthy pregnancies. But because the fix is so simple (just start the night on your side), it’s considered a worthwhile precaution.
What If You Wake Up on Your Back
This is one of the most common worries, and the answer is straightforward: don’t panic. Briefly ending up on your back, even for an hour or two, is unlikely to cause harm. Your body has built-in warning signals. If back sleeping is reducing your blood flow significantly, you’ll typically feel it: shortness of breath, a faster heartbeat, lightheadedness, or just general discomfort. These sensations usually wake you up and prompt you to roll over before any real problem develops.
What matters most is the position you fall asleep in, because that’s the position you spend the longest stretch of the night in. If you drift onto your back during sleep and wake up there, simply roll to either side and go back to sleep.
Practical Tips for Staying on Your Side
Side sleeping doesn’t come naturally to everyone, especially if you’ve spent years as a back or stomach sleeper. A few simple tools can help you stay in position through the night.
- Wedge pillow behind your back: This is one of the simplest solutions. Tucking a small wedge behind you creates a gentle barrier that prevents you from rolling flat. It works well for habitual back sleepers because it doesn’t feel confining.
- C-shaped or U-shaped body pillow: These wrap around your entire body, supporting your bump, back, and the space between your knees simultaneously. They’re bulkier but help maintain a comfortable side-lying curl throughout the night.
- Pillow between the knees: Even a regular pillow placed between your knees takes pressure off your hips and lower back, making side sleeping more comfortable and easier to sustain. A memory foam knee wedge works well on smaller beds.
If you’re a back sleeper making the transition, start practicing side sleeping before the 28-week mark so it feels more natural by the time it matters. Placing a pillow under your bump can also relieve the pulling sensation that makes some people instinctively roll onto their backs.
The Bottom Line on Sleep Position
The traditional advice to sleep on your left side isn’t wrong. It does optimize blood flow. But the more important message from current research is simpler: after 28 weeks, fall asleep on either side. Left or right makes no measurable difference in outcomes. The position to avoid is flat on your back for extended periods, and even then, your body will usually alert you before it becomes a problem.