Yep, they vaxed "back in the seventies", too.
Though for her to be a teen in the seventies she'd've been born in the sixties, or even late fifties, but the mumps vaccine has been around since at least the early 50s.
Yeah, the story has changed SLIGHTLY since I made this post (due to time constraints, I've moved the first story forwards to '83. She's 17 then, so she would be born in '66). Although I know "never had periods ever" is an option, a friend of the family, whose in her mid 40's now, never had a period until she was 17, and, although in circumstances like
Carrie, it is possible for it to go on until 16-17. I need to convince the reader.
She needs to know fairly definitively that she's not capable of getting pregnant due to circumstances prior to the novel, but she gets pregnant in horror-movie circumstances (it's a horror novel - basically a succubus plants her baby inside her), so it's enough for her to freak out and know that some creepy shit is afoot. However, I'm also not sure about removing her uterus because she has to be physically able to carry the baby to term, although it can nearly kill her and be a totally freak incident. (Does this make any sense?) A severe STD would work well for these circumstances, as her boyfriend already has suspicions of her fidelity, although he has to not know she had an STD when she was admitted to hospital. I suppose she could always lie, although I know the doctors probably wouldn't be happy about that.
Sorry if that's totally confusing.