In Zoe Oldenbourg's The Crusades, she mentions that a medieval baby's chances of surviving into adulthood were 1 in 4. And this was an accepted fact of life that spanned across feudal classes.
Is this true? I mean, with infant diseases, the ravage of annual plagues, and general accidents, this seems feasible. But are there additional sources confirming this?
The honest truth is that we have no idea. I'd be intruiged to see the source she uses for that, because as far as I'm aware there's nothing (at least up to the Black Death) that might allow us to make such an assertion. You can do best guesses, which is what I assume she's done there, but nothing more definite.