For sleeping on their back. Our youngest daughter would not sleep on her back. She still sleeps on her tummy with arms folded under her. The only way she sleeps otherwise is in the car seat when properly exhausted. Put her to bed and she will turn around and fall asleep. So what does medicine say then?
Depends on the kid too. All 3 of my kids could lift their head up when they were born. A couple of times I forgot all babies aren't like that and picked up a friend's baby without adequate support.
It says that there is like a 10x risk of SIDS in the first four months of life with tummy sleeping.
I don't agree with her on everything, but Emily Oster's chapter on SIDS (in the second book I think, Cribsheet) I think does a good job outlining the data on it. And my brother just had a kid who also would absolutely not sleep on his back. Once he could roll he just sleeps on his tummy (but once they can roll SIDS is not really an issue)
Sleeping on the back only matters when they are very young. It’s for when they don’t have the strength to turn themselves out of a face-down / suffocating position. That’s why you practice tummy time (neck/head lifting) with an infant. Once they are older they can sleep how they like