Expectant moms get a lot of advice from the Internet, friends, and even strangers. It's a lot of judgment, given that this miracle of birth has already happened 13 billion times without anyone telling mothers to eat purple vegetables or their child won't get into Stanford.
The digital age, where publication is cheap, coupled with epidemiology which can show almost anything, has led to a lot of confusion. But when the word "stillbirth" is invoked, as was recently down in The Journal of Physiology, pregnant women will panic.
Yet they shouldn't. No babies were harmed in a study which nonetheless claims babies could be harmed if moms sleep on their backs.
After monitoring 29 pregnant women at 34-38 weeks gestation, using ECG in their homes, all of them healthy (not obese or smokers) with healthy babies and recording the mother's position during sleep, the authors correlated the mother sleeping on her back with a greater risk of stillbirth. That's scary for any parent.
Their argument: When the mother slept on her back, the fetus was less active. Fetal activity is one measure of its wellbeing. Fetuses were only in an active state when the mother was on her left or right side. When the mother changed position during sleep, for example from her left side to sleeping on her back, the baby quickly changed activity state and became quiet or still.
All of the women had perfectly healthy babies so all the authors went by was changes in activity by the baby and then invoking other studies which said they correlated sleep position to risk of stillbirth. It's not even a proxy, it's a proxy of a proxy.
On Twitter, any number of people are going to rush to repeat this and tell pregnant women they have one more thing to be in a panic about. Yet the evidence is lacking and the epidemiological risk is approaching your chances of hitting the lottery. It's probably more comfortable for pregnant women to sleep on their side, but when evidence is not presented, next they could be told which side that is. And it will only be valid in the world of statistics.