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human
ELI5: Why aren't babies allowed to sleep on their side?

My girlfriend is about 34 weeks pregnant with our first child and she recently asked me if I knew what position you're supposed to lay babies down in to sleep. I said that I thought you lay them on their side, so that if they were sick in the night they wouldn't choke, but she told me that babies were in fact only allowed to sleep on their backs.

I obviously believe her and since going to antenatal classes they also said the same thing, so without question I will lay our baby on his back to sleep. I still feel a little bit anxious about doing it though. Maybe I'm thinking of the situation in adult terms too much, but I just can't seem to shake the worry that, similar to an adult who'd passed out, if he's sick while lying on his back he might choke.

Please can you explain why it's safer to lay him on his back and why it's unlikely he'll choke if he's sick in the night/during a nap.
gpt
Babies can't really lay on their sides.  They're pretty round and have little control over their limbs, so if you lay them on their side, they pretty quickly roll either to their back or to their front.  So your only options are on their back or on their front.

The evidence is overwhelming that babies are way less likely to die from SIDS if they sleep on their back.  It's not clear why that is (mostly because we won't experiment on babies and watch them die to figure out why, for obvious reasons); the leading theory is that babies on their front can end up with their mouths and noses in a position where they can't breathe and they're not strong enough to clear them.

Overall, being sick while on their back is actually not a big concern.  They have a functioning gag reflex which makes them spit out anything that gets in their throat.  The problem with adults is usually being drunk, which impairs the gag reflex and makes it possible to choke if liquid ends up in their throat.  Unimpaired adults don't normally choke on their own vomit either.