How Important Is Side Sleeping When Pregnant?

Pregnancy makes you tired. When I was pregnant with Penelope, I was so tired in the first trimester that I bought a napping couch for my office (it took a long time to arrive, but came in handy in trimester three). And as the pregnancy progresses, it gets increasingly hard to get comfortable. The bed starts to fill with oddly shaped pillows, blankets, and wedges. It is lucky that we upgraded to a king bed eventually, or I’m sure Jesse would have been sleeping on the floor at some point, given the wide variety of sleep accompaniments I purchased.

It therefore adds insult to injury to be told that sleep position is restricted. Pregnant people are told not to sleep on their back, which, if you’re someone who typically sleeps on their back, can be hard to get used to a new position. And waking up throughout the night if you inevitably end up on your back leads to the dreaded middle-of-the-night panic Google. But where does this recommendation come from, and is there any truth to it?

Take me to the bottom line

Where does the side sleeping recommendation come from?

The theory behind pregnant people being told they aren’t supposed to sleep on their back is that as the uterus gets larger (beyond 20 weeks or so), it can compress an important blood vessel (the vena cava). This decreases the mother’s blood pressure and can reduce blood flow to the placenta and baby. It’s true that this does happen, but the more relevant question for you is whether there is any evidence that this actually has risks for the baby. If you wake up on your back, should you worry?

As it turns out, the best evidence suggests no.

There are various ways one can try to figure this out. The first is to interrogate basic biology. That is, since there is a clear biological mechanism (reduced blood flow), we could test to see how important that is. How much is blood flow really restricted?

In one very nice study of this, researchers had women lie on their backs, and they measured the blood flow to the uterus. They found that lying down has no particularly bad impact on blood flow. A couple of women in that study became uncomfortable, but they felt better when they changed positions. The authors conclude that some people might be uncomfortable sleeping on their backs, but if you are not one of them, you should feel fine about it.

This paper was included as part of a review article about the topic. Their conclusion, which I think is clear and succinct, is as follows:

“… advising women to sleep or lie exclusively on the left side is not practical and is irrelevant to the vast majority of patients. Instead, women should be told that a small minority of pregnant women feel faint when lying flat. Women can easily determine whether lying flat has this effect on them, and most will adopt a comfortable position that is likely to be a left supine position or variant thereof. […] since finding a comfortable position in bed in late pregnancy is not easy, physicians should refrain from providing impractical advice.”

The review above concluded that the no-back-sleeping recommendation doesn’t make sense.

Despite this, there were still concerns. For example, a study published in 2011 showed a link between maternal sleep position and stillbirth. In this study, researchers identified women who had a recent stillbirth and interviewed them about their behaviors while pregnant. They also spoke to some women who had a healthy baby. They found that one behavior that was more common in the stillbirth group was back sleeping. This type of study, which is called a case-control study, can be informative, but it’s subject to a lot of concerns. The two groups are recruited very differently, and it’s hard to know whether their recall of their behaviors in the past is correct.

However, without better data, this study and earlier ones like it generated an “abundance of caution” recommendation from at least some providers, which is where this all-or-nothing advice came from.

Fortunately, more recent data has pushed back against this abundantly cautious recommendation and reinforced the view that any sleep position is fine.

In particular, in 2019, a study came out on this topic. The key innovation in this study was that it was prospective. The authors recruited 8,706 women who were pregnant and had them complete questionnaires in the first trimester and early in the third trimester about their sleep position. They then observed what happened and coded whether there were any “adverse outcomes” in the pregnancy. This included stillbirth, but also infants who were small for gestational age or had other issues.

The authors find no association between any adverse outcomes and sleep position either at the first visit in early pregnancy or the third visit in mid-pregnancy. Very unfortunately, there were some stillbirths, and there were many more less serious complications, but these did not seem to be linked to sleep position.

Putting this together with the evidence at the top of this section on minimal changes in blood flow, along with the conclusions of that review article, the bulk of the evidence seems to reject the link between sleep position and stillbirth or other negative outcomes. It seems likely that the association we see in the case-control data is an effect of reporting. In the end, the best data suggests that back sleeping at any point in your pregnancy is fine.

Does sleep position matter at all?

So if it’s okay for you to sleep on your back, what about other positions? Sometimes people will hear they should sleep on their left side and not their right side. Good news: the same data that shows the safety of back sleeping also shows the safety of sleeping on either side. The main findings are simply that sleep position doesn’t link to stillbirth.

Sleeping on your stomach is also fine, as long as you can do it. As you get more pregnant, it may simply be impossible to keep doing this.

Closing thoughts

For me, one of the most infuriating things about insomnia when I was pregnant was when people would tell me things like “You think you’re tired now, wait until the baby comes!” And it is unfortunately true — however little sleep you are getting at the end of pregnancy, you’ll probably get less once the baby arrives.

But there is a silver lining: when you do finally get a chance to sleep, there are none of those pregnancy aches and pains that kept you up. The quality of sleep goes up a lot, even if the quantity does not.

The bottom line

Based on the best available evidence, although there are small changes in blood flow, there is no conclusive connection between back sleeping and adverse outcomes.
As pregnancy progresses, most pregnant people will eventually stop sleeping on their backs anyway, as it can get uncomfortable. But there’s no need to worry if you wake up on your back after falling asleep on your side.
The same data concludes that there are no links to adverse outcomes for any sleeping position — one is not better (or worse) than the other. You can sleep on your right or left side, and it is even safe to sleep on your stomach, although that may get more uncomfortable as your pregnancy progresses.