A Pharmacist’s Warning: The Sleeping Position That Increases Stillbirth Risk by 2.6x
My wife is 15 weeks pregnant. She was asleep next to me when I finished reading a research paper — and I just sat there staring at the ceiling for a while.
I'm a pharmacist with nine years of experience. Reading studies is basically a professional habit. But this one hit differently.
"The risk of stillbirth is 2.6 times higher depending on how you fall asleep?"
This post is about a large international study that pooled data from 3,108 pregnant women. The science isn't complicated. The takeaway is simple. But too many people have never heard of it.
Choosing the right sleeping position after 28 weeks can significantly reduce the risk of late stillbirth.
What Study Are We Talking About?
This is a 2019 international collaborative study led by researchers in New Zealand. It goes by the name CRIBSS — Collaborative Individual Participant Data of Going-to-Sleep Position and Stillbirth. Here's the plain-English version of what it did:
Researchers gathered raw data from five separate large-scale studies around the world and combined them into one mega-analysis, specifically looking at the relationship between a pregnant woman's going-to-sleep position and the risk of late stillbirth (losing a baby after 28 weeks).
The key word is "individual participant data." This wasn't just combining published results — researchers pulled each patient's raw data and re-analyzed everything from scratch. That makes it one of the most reliable types of evidence in medical research.
The study included 851 mothers who experienced a stillbirth and 2,257 mothers who had healthy pregnancies — 3,108 women in total.
The Core Finding: Back Sleeping Is Risky
Let's start with the bottom line.
After 28 weeks of pregnancy, falling asleep flat on your back (facing the ceiling) is associated with a 2.6-fold increase in the risk of late stillbirth.
This held true even after researchers adjusted for age, weight, smoking, and pre-existing health conditions. In other words, with everything else being equal, sleeping position alone was associated with a 2.6x difference in risk.
Here's how the numbers break down:
| Going-to-Sleep Position | Stillbirth Risk (vs. Left Side) | Statistical Significance |
|---|---|---|
| Left side | Reference (1.0×) | — |
| Right side | 1.04× | No meaningful difference |
| Back (supine) | 2.63× | Statistically significant |
Left side or right side — it doesn't matter. The only position that stood out as risky was lying flat on your back.
Why Is Back Sleeping Dangerous in Late Pregnancy?
This is the part I was most curious about as a pharmacist. Here's how the study explains the mechanism.
In the third trimester, the uterus grows large enough that when a pregnant woman lies on her back, it compresses two major blood vessels running along her spine:
- The inferior vena cava — the large vein that carries blood from the lower body back to the heart
- The aorta — the large artery that sends blood from the heart to the rest of the body
MRI imaging conducted at the University of Auckland found that lying on the back reduced blood flow through the inferior vena cava by up to 80%. The body tries to reroute blood through other vessels, but it can't fully compensate. Cardiac output drops by roughly 10–25%, and blood flow to the uterus and placenta decreases along with it.
A healthy baby can usually tolerate this. But what about a baby that's already measuring small, or one that's already under some form of stress? That baby may not be able to handle hours of reduced blood flow through the night.
In my pharmacy, I occasionally get questions from pregnant women who feel dizzy or short of breath when they try to rest on their back. Most assume it's just pregnancy discomfort. But this is likely the same mechanism at work — their body signaling that blood flow is being restricted. The study found that when a small baby and back sleeping occurred together, the stillbirth risk jumped to approximately 16 times higher than baseline. Each factor is independently risky — combined, they multiply.
"Do I Have to Sleep Only on My Left Side?"
This question comes up constantly in pregnancy forums. An older study had suggested the left side was better, so many pregnant women have been stressing themselves out trying to stay on their left all night.
The CRIBSS study is clear on this point:
It doesn't matter whether you sleep on your left side or your right side. Either one is fine. The only thing to avoid is lying flat on your back.
The adjusted risk ratio for right-side sleeping was just 1.04 — essentially identical to the left. Statistically, there was no meaningful difference between the two sides.
This matters because telling pregnant women they must sleep on their left side creates unnecessary stress, and stress itself disrupts sleep quality. Sleep on whichever side is comfortable. Left or right — your call. Just avoid being flat on your back when you fall asleep.
What If I Roll Onto My Back During the Night?
This is the most practical concern — and the first thing my wife asked when I walked her through this study. She immediately said, "But I always end up on my back by morning." Fair point.
Here's what the research team says:
If you wake up and find yourself on your back, simply roll to your side. That's all you need to do.
The study specifically measured going-to-sleep position — not the position you end up in throughout the night. People tend to stay in their starting position for the longest stretch of sleep, which is why that initial position matters most. Rolling around during sleep is normal and beyond your control. Waking up and repositioning yourself is enough.
How Much of a Difference Could This Actually Make?
The research team ran the numbers on population-level impact:
If all pregnant women over 28 weeks fell asleep on their side, it could prevent approximately 5.8% of all late stillbirths worldwide — an estimated 153,000 babies per year.
5.8% might sound modest. But that's 153,000 lives. And this intervention costs nothing — no medication, no equipment, no appointments.
To put it in context, here's how sleeping position compares to other known risk factors for late stillbirth:
| Risk Factor | Population-Attributable Risk (PAR) |
|---|---|
| Fetal growth restriction (small baby) | 30.6% |
| Reduced fetal movement | 17.1% |
| Obesity | 12.1% |
| Smoking | 8.7% |
| Overweight | 8.2% |
| Back sleeping (supine) | 5.8% |
Most of those other risk factors are difficult or impossible to change once you're pregnant. Sleeping position? You can change that tonight.
Can Women Actually Make This Change?
A survey of pregnant women in New Zealand found that more than 80% said changing their going-to-sleep position would not be difficult if it was good for their baby.
If you're worried about rolling onto your back during the night, try placing a pillow or rolled blanket behind your lower back. It creates a gentle physical barrier without any effort once you're asleep. No gadgets needed — whatever pillow is already on your bed works fine.
Why 28 Weeks Specifically?
28 weeks is the threshold the World Health Organization uses to define "late stillbirth." It's also the point at which the uterus has grown large enough that lying on your back causes meaningful compression of the major blood vessels. Before that stage, the physical pressure simply isn't significant enough to have the same effect.
28 weeks marks the beginning of the third trimester — roughly seven months pregnant. If you're not sure exactly where you are in your pregnancy, it helps to have a clear picture of how the trimesters are structured. I put together a full breakdown here: Pregnancy Weeks, Trimesters, and Key Precautions: What Every Expectant Parent Should Know.
My wife is at 15 weeks, so we have some time. But building the habit early makes the transition effortless later.
The Three-Sentence Summary
After reading the research, here's exactly what I told my wife:
1. From 28 weeks onward, fall asleep on your side. Left or right — doesn't matter.
2. If you wake up on your back, just roll to your side. That's enough.
3. Same goes for naps. Start on your side.
No medication. No supplements. Just position. As a pharmacist, it honestly surprised me that something this simple is this directly connected to a baby's safety.
Of course, sleeping position isn't the only thing that matters during pregnancy. Regular prenatal checkups, paying attention to fetal movement, and being careful about what you take are all still important. If you're unsure which medications are safe to use during pregnancy, I've written a pharmacist's guide on that too: Safe Medications During Pregnancy: What You Can Actually Take. But if there's one thing to change starting tonight, this is it — the easiest and most immediate step you can take.
Frequently Asked Questions
Is it really dangerous to sleep on your back while pregnant?
According to the 2019 CRIBSS meta-analysis, falling asleep on your back after 28 weeks is associated with a 2.63-fold increase in late stillbirth risk. The likely cause is compression of the inferior vena cava and aorta, which reduces blood flow to the uterus and placenta.
Is sleeping on the right side safe during pregnancy?
Yes. The same study found no statistically meaningful difference between left-side and right-side sleeping. Both are considered safe. The only position associated with elevated risk was lying flat on the back.
What if I roll onto my back while sleeping?
Don't panic. Simply roll back to your side when you wake up. The study focused on going-to-sleep position, which is the position you maintain for the longest continuous stretch. You can't control what happens once you're deeply asleep, and repositioning when you wake is sufficient.
When should I start sleeping on my side?
The research specifically covers the period from 28 weeks onward (the third trimester), when the uterus is large enough to compress major blood vessels. That said, forming the habit earlier is harmless and makes the adjustment easier later on.
What else can I do to support a healthy pregnancy?
Sleeping position is just one piece of the puzzle. Prenatal supplements taken on the right schedule, avoiding certain medications, and understanding what's safe to eat and drink all matter too. If you're navigating supplement timing, this guide may help: 9-Year Pharmacist's Guide to Prenatal Supplements: Safe Schedule & Tips.
References
① Cronin RS et al. (2019). An Individual Participant Data Meta-analysis of Maternal Going-to-Sleep Position, Interactions with Fetal Vulnerability, and the Risk of Late Stillbirth. eClinicalMedicine.
→ Read the full study
② The Conversation (2019). Mega study confirms pregnant women can reduce risk of stillbirth by sleeping on their side.
→ Read the article
Disclaimer: This post is written by a licensed pharmacist for general informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN, midwife, or healthcare provider with any questions about your pregnancy. Every pregnancy is different, and your care team is best positioned to give you personalized guidance.
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