As longtime readers of my blog know, in 2011, carrying my first child, I became obsessed with the question of whether pregnant women could lie on their backs–either for short periods of time, such as during a yoga class, or while asleep at night.
Several OBs told me to avoid lying on my back. But their justifications were murky, and their advice conflicting. Not a one could point to a single published study backing this advice up. And when asked at what point in pregnancy I needed to start avoiding back sleeping, their answers were all over the place. One told me it was verboten from 4 months on, another from 5 months on, and the third claimed I should worry only in the last month or so.
Sleeping with a bowling ball-sized stomach is challenging, to say the least. At the same time, groundless sleep prohibitions with vague but terrifying warnings that you might harm your baby are immensely frustrating, and yet almost impossible to disregard.
But while in 2011, these dire warnings sounded like a yet another pregnancy prohibition in search of a reason (no Brie, not a drop of alcohol, keep your heart rate below 140 while exercising, etc., etc.), several studies (one from Australia, two from New Zealand, one from Ghana, and the latest, from the UK) have since found that back sleep may indeed be linked to what is termed late stillbirth, or pregnancy loss after 28 weeks.
The science of back sleeping and stillbirth
The latest and largest of these studies, involving over 700 stillbirths, came out earlier this year. Like its smaller predecessors, this UK-based study compared the sleep practices of women with a late stillbirth with those of women with ongoing pregnancies.
(Why did researchers use a retrospective study design? Because late stillbirth is fairly rare, affecting only around 3 pregnancies in 1,000. Prospective studies on late stillbirth–that is, following a large number of pregnant women before their pregnancy outcomes are known– while scientifically superior, are prohibitively large and expensive.)
As in the earlier studies, women who slept on their backs had about twice the chances of late stillbirth than women who slept on their left sides. Because it was the larger than earlier studies, the researchers were able to look at whether back sleeping was only risk factor for some women, such as those who were obese or who had a baby that was small for its gestational age (SGA). This was not true–back sleeping appeared to increase the risk of stillbirth for all women.
Why would back sleeping cause stillbirth?
A large vein, the vena cava, runs under the right side of the uterus. Late in pregnancy, when a woman lies on her back or her right side, the unusually heavy weight of uterus can compress this vein, reducing blood flow to the fetus.
Researchers believe the reduction of blood flow to the fetus, if sufficiently severe and prolonged, may lead to fetal death.
What does this mean in practice?
First off, if you have been lying on your back to sleep, do not panic! Your baby is fine.
But should you continue to sleep on your back? What if you accidentally roll onto your back while sleeping? What if you cannot comfortably sleep in any other position?
Well, here’s where it gets complicated. With four studies all showing the same thing, it seems likely that back sleeping really does raise the chances of a late stillbirth. But whether it meaningfully raises the chances of a late stillbirth is a more complicated question.
First, let’s recall that late stillbirth is rare–affecting only 3 pregnancies in 1,000. About a third of these late stillbirths are caused by birth defects, placental abruption (the placenta breaks away from the uterine wall), and infections. Back sleeping is unrelated to these explained stillbirths.
For the remainder, known as unexplained late stillbirths, the causes remain incompletely understood, but primarily appear related to a failing placenta. Back sleeping may exacerbate placental failure by further restricting the fetus’s already compromised blood flow.
However, back sleeping is far from the most important risk factor linked to unexplained late stillbirths. Being obese, being over 40, and smoking during pregnancy are far more important. The Centers for Disease Control estimates that in high income countries like the U.S., being obese or overweight during pregnancy contributes to about 8,000 stillbirths each year, pregnancy among women aged 35 and up contributes to about 4,200 stillbirths each year, and smoking during pregnancy contributes to about 2,800 stillbirths each year.
About 45% of pregnancies that end in late stillbirth involve fetuses with intrauterine growth restriction. Mom having diabetes or high blood pressure also ups the risk. And African-American women have a higher risk than Asian and White women.
In short, back sleeping appears to contribute to a fraction of a fraction of an absolutely tragic, but thankfully rare event.
And here’s what does not make it into the standard media write-ups about this research: Regular daytime napping also raises the risk of late stillbirth about two-fold, according to these studies. No one knows why. I have yet to see a researcher even hazard an explanation.
Oh, and so does getting up to go to the bathroom once or less in the middle of the night. (Yep, you can’t even consider not having to pee multiple times per night in your third trimester as a win! There is no winning with these data.)
As yet, not a single researcher or public health official has proposed that pregnant women abstain from napping or that they set alarms to ensure they get up to pee at least twice per night. Perhaps they recognize the futility of such efforts. Women in their third trimester are tired. Then again, they are pondering widespread public health messages again back sleeping.
Counting on these people to consider the practical effects of their pregnancy recommendations is like counting on a newborn to sleep through the night. In short, don’t.
The Bottom Line
Fortunately, whether this research has you shopping for a new pregnancy pillow, vowing to sleep only on your left side, or calling BS as you reach for a second helping of Brie and a half glass of wine, a very simple fix exists: Put a pillow under your right side if you sleep on your back. Propping your right buttock up as little as 10-15 degrees prevents compression of the vena cava. And women who sleep on their backs but propped up have no higher risk of stillbirth than women who sleep on their left sides, according to this study.
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