She sounds, I'm sorry to say, VERY immature, are you sure she's not too young for you? Don't get me wrong, I don't want to start a 'how young is too young' thing, and I'm not saying physically (that'd make me a bit of a hypocrite-I started going out with my boyfriend when he was 19 and I was 16), but mentally. The point is mainly on the sex side of things. I went on the pill a few months into the relationship, I didn't think I'd remember it either, but it's surprisingly easy to.
By being too lazy to bother taking the pill or anything, she's not only jeopardizing her future, but yours too-do you want to be a father at 19? In a relationship which I honestly don't think is going to last.
Ok, so if you do have an 'accident' (are you actually sure that it's not just that she WANTS to get pregnant), there is always the option of abortion, but from what you've said, I don't think that she'd go through with that. I think you should seriously reconsider your relationship with this girl.