in breaking dawn they get married and then on the honeymoon they do the deed
which results in bella getting pregnant, cause shes stil human and obviously edward isnt, the baby is half immortal, so it can grow up (but at incredible speeds) but then stays at an adult age. also because the baby is half vamp while bella is carrying all the baby wants is blood and is killing bella from the inside out. so when bella eventually gives birth shes dyin and therfore gets turned into a vamp. its a fantastic book and i reccomend reading it, theres so much more but im not sure how much exactly u want to kno lol xx