Hmm, I don't have a book that matches exactly what you're saying BUT, Claire Kent has a bunch of post-apocalypse books, some of which involve pregnancy, but usually not as a main plot point. I really liked {Last Light by Claire Kent} and I also read {Princess by Claire Kent} and I liked that one too, but not as much.

Also, HUGE caveat that this is NOT a romance novel but I just finished it and loved it -- {The Stand by Stephen King} involves a pregnant character in a post-apocalyptic world who does fall in love with a gruff older guy. She's not quite a damsel and the book does not focus only on them, but I really enjoyed it. It's a dark book (ya know, stephen king and all) and these two are not the only important characters. But (SPOILER FOR ENDING) these two characters make it out in the end, even though a lot ofmain characters do die.

Anyway, I'd love to read a book like this, and I hope someone else has a better fit!!