Premature is a medical term that's a little hard with someone with an intellectual disability to parse unless they're already familiar.

If you don't have that knowledge retained it's a much larger leap to figure out what "premature" means. you have to understand the prefix (pre = before) and know what "mature" means in this context (not old, but rather "not ready").

This complicates the fact that premature birth also requires a lot of prior understanding (why can a baby be born too early? why is that a problem?)

Perhaps the author could have elaborated a little with plain language... "Born too soon" might be clearer.