Baby on the way? You've probably heard you should cozy up on your side (not your back) every night. Learn where that recommendation comes from, plus whether or not it's something you even need to worry about.

By Cassie Shortsleeve
Photo: Getty Images / Artem Varnitsin / EyeEm

During pregnancy, there are all sorts of health rules you have to follow: No deli meat, careful with the seafood, choose your abs exercises wisely (*ugh*), and say goodbye to Botox or retinol. But, turns out, there are even rules for when you're sleeping! A big one you've likely heard: If you have a baby on the way, avoid sleeping on your back and choose side-sleeping positions instead. (Related: Safe Sex Positions During Pregnancy That Still Offer Serious Pleasure)

The idea is that the added weight from your uterus can compress a large vein in your back called your vena cava. The vein is responsible for bringing blood from the lower half of your body to your heart, so theoretically, if you compress it, you could have diminished blood flow to your heart (read: less for you and for baby). Think of it as a rock on a hose, says Allison Boester, M.D., ob-gyn at Weill Cornell Medicine and NewYork-Presbyterian.

And some studies-including, most recently, a 2017 paper published in the British Journal of Obstetrics & Gynecology found that women who reported sleeping on their backs were twice as likely to have a stillbirth as those who slept on their left sides-make this sleeping position seem seriously off limits.

How bad is sleeping on your back while pregnant, really?

Well, first off, not all docs are convinced that these concerns are 100 percent justified, says Martha Monson, M.D., a clinical fellow in the University of Utah's Maternal-Fetal Medicine Division. Sleep position also is not exactly something many women have great control over, adds Lauren Theilen, M.D., an assistant professor of obstetrics and gynecology at the University of Utah Health.

It's possible that sleeping on your back for prolonged periods of time during pregnancy could negatively impact oxygen levels to your baby, but other issues (disordered breathing or obstructive sleep apnea) may be the real culprit, says Dr. Monson. "And none of the currently published studies on this topic have addressed this possibility."

As for the 2017 study mentioned above, it was also done retrospectively, notes Dr. Monson. Meaning researchers asked women who had tragically experienced a stillbirth, weeks after the fact, how they slept before the incidence. It's a kind of reporting that can lead to recall bias (a.k.a. you might not remember everything 100 percent accurately), she says.

Another important thing to remember? All of this advice about back sleeping would refer to a time in pregnancy when your uterus is big enough to even potentially compress the cava vein in the first place, says Dr. Boester, who adds that this would be more toward the end of pregnancy. "People ask me about this issue at about 14 weeks, but you wouldn't have the effect until the belly is much larger."

Also, if you were sleeping on your back and obstructing the vena cava? You'd likely feel lightheaded or uncomfortable, which would probably prompt you to move, she says.

That said, that there *is* something about being on your side versus your back during labor, says Dr. Boester. Sometimes doctors tilt women (especially women who have had an epidural and might not be aware of any symptoms they might be experiencing) from being flat on their back. But again, lying in bed sans an epidural, you'd likely feel uncomfortable and move before anything worrisome could happen. "Your body will tell you if something doesn't feel right," she says.

The Bottom Line

As far as official recs go, The American Pregnancy Association notes that sleeping on your back can "cause problems with backaches, breathing, the digestive system, hemorrhoids, low blood pressure, and cause a decrease in circulation to your heart and your baby." And the American College of Obstetricians and Gynecologists (ACOG) recommends sleeping on your side (though they note this is a way to avoid back pain).

But first and foremost, it's important to sleep in a position that's comfortable for you-and for most pregnant ladies that just so happens to be on their side (with a pregnancy pillow to help support you!). Sleeping on your back toward the end of pregnancy usually simply just isn't the most comfortable position in the first place thanks to all of the added weight on your tummy, says Dr. Boester.

Otherwise? "I don't make it a rule to not sleep on your back," says Dr. Boester, who notes that pregnancy pointers like avoiding unpasteurized cheese or raw meat are far more important than sleeping position. (Related: Pregnancy Sleep Tips to Help You Finally Get a Solid Night's Rest)

If you're worried, try to stick to sleeping on your left side. The vena cava runs up your back slightly on your right side, so sleeping on your left avoids it entirely.

And as far as the risk for stillbirth goes, while tragic, there are other things-obesity, smoking, drug use, high blood pressure, and diabetes-that are far more in your control and can lower your risk more so than sleeping position, says Dr. Theilen. And if you think you might have a sleeping disorder such as sleep apnea? Make sure to talk to your doctor.


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