Pyometra is also something else that can kill them if they're not spayed
ACVS - Pyometra in Dogs & Cats
Myself, I think there are too many people wanting to breed just to be breeding, or for money, or because their pup is cute. My pups are cute too, and all of them have been spayed except for the youngest, and it's only because she's still too small. My advise(and I'm not saying this mean) is to get her spayed before she comes in to her first heat. Might save you a lot of heartache in the end