they're probably safe to take, but things do start to break down after expiration, so you won't know how much of anything you're getting -- maybe not enough of everything (at least not enough according to what the package says you are getting, i mean).
that being said, prenatals aren't even really all that necessary if you are eating well. so if you consider them as just a supplement, you're probably not missing out on all THAT much. and i'm assuming that you don't have tons and tons left over, just a month's worth or so? so you'd be getting new ones soon anyway?