Most guidelines recommend stopping back sleeping at 28 weeks of pregnancy. That’s the point where research shows a meaningful increase in the risk of stillbirth and lower birth weight associated with falling asleep on your back. Before 28 weeks, the uterus isn’t large enough to cause significant problems in the supine position.
Why 28 Weeks Is the Cutoff
As pregnancy progresses, the uterus grows heavy enough to compress a major blood vessel called the inferior vena cava when you lie flat on your back. This vein runs along your spine and carries blood from your lower body back to your heart. When it’s compressed, less blood returns to your heart, which means less oxygenated blood reaches both you and your baby.
Before the third trimester, the uterus simply isn’t large or heavy enough to create this compression. By 28 weeks, the combined weight of the baby, placenta, and amniotic fluid becomes significant enough that lying flat can reduce blood flow in a clinically meaningful way. This is why the guidance specifically targets 28 weeks rather than an earlier point in pregnancy.
How Back Sleeping Affects You and Your Baby
The compression of that major vein can trigger a set of symptoms sometimes called supine hypotensive syndrome. You may feel dizzy, nauseated, sweaty, or notice your heart racing. Your blood pressure can drop by 15 to 30 points. These symptoms are temporary and resolve as soon as you shift to your side, but they’re a signal that blood flow has been compromised. Some women experience something closer to a fainting spell, which is usually enough to make them avoid the position instinctively going forward.
The effect on the baby is measurable too. Studies using fetal pulse oximetry during labor found that lying on the back was associated with lower fetal oxygen levels compared to lying on the left side. A large New Zealand case-control study found that going to sleep in the supine position was associated with a 3.7-fold increase in late stillbirth risk, independent of other common risk factors. The researchers estimated that supine sleep position accounted for about 9.4% of late stillbirths in the population, making it one of the more significant modifiable risk factors.
Left Side vs. Right Side
For years, left-side sleeping was considered the gold standard because the inferior vena cava sits slightly to the right of the spine, and lying on the left theoretically keeps pressure off it entirely. However, the most recent evidence reviewed by the UK’s National Institute for Health and Care Excellence (NICE) compared outcomes primarily between back sleeping and side sleeping. The guidance from NSW Health in Australia states clearly that either side appears equally safe. So if you’re more comfortable on your right side, that’s fine. The key distinction is back versus side, not left versus right.
What If You Wake Up on Your Back
This is the single biggest source of anxiety for pregnant people searching this topic, and the answer is reassuring: simply roll back onto your side. The position you fall asleep in is the one you hold for the longest stretch of the night, so that’s the position that matters most. Waking up on your back occasionally doesn’t mean you’ve harmed your baby. Your body often alerts you to the problem before it becomes dangerous, through dizziness, discomfort, or restlessness that wakes you up in the first place.
You can’t consciously control what happens while you’re asleep. What you can control is the position you settle into when you drift off. That’s where the focus should be.
Do Pregnancy Pillows Actually Help?
Many people assume a pregnancy pillow will keep them from rolling onto their back at night. A randomized crossover trial tested a U-shaped pillow specifically designed to prevent supine sleep, with one firm arm behind the back and a softer arm supporting the belly. The result was disappointing: the pillow did not reliably prevent or reduce back sleeping, and women using it still spent an average of one hour per night on their backs.
That doesn’t mean pillows are useless. A pillow between your knees or tucked under your belly can make side sleeping more comfortable and help you fall asleep in the right position. Some people find that placing a firm pillow or rolled towel behind their back creates just enough of an obstacle to make full back sleeping uncomfortable, even if it doesn’t prevent it entirely. The goal isn’t perfection. It’s making side sleeping your default position at the start of the night.
Practical Tips for the Third Trimester
If you’ve been a back sleeper your whole life, switching at 28 weeks can feel unnatural. A few strategies can help:
- Start practicing early. If you begin side sleeping in the second trimester, it will feel more natural by the time it matters.
- Use pillows strategically. A pillow between the knees reduces hip strain. One behind the back discourages rolling over. A wedge under the belly supports its weight.
- Try a slight tilt. You don’t need to be perfectly perpendicular to the mattress. Even a partial lean to one side, supported by a pillow, is enough to take pressure off the vena cava.
- Don’t stress about overnight movement. Focus on your going-to-sleep position. If you wake up on your back, roll over and go back to sleep.
The 28-week mark is a guideline, not a cliff edge. The risk from supine sleeping increases gradually as the uterus grows, and your own body will often tell you something is wrong before any serious harm occurs. The most effective thing you can do is make side sleeping a habit for the start of each night.