How can one become a vampire at birth? Please explain?

My family and I watched blade ( the tv-14 version) last night and a interesting conversation came up? My son that is 9 said " This is crazy, how is it possible for someone to be born a vampire as a baby if the birth parents are technically dead, and I was told vampires don't grow old, so wouldn't the baby vampire be a baby forever? I agree with him! He is not talking about Blade but the others in the movie that state they where born full vampire. With blade himself it is understandable (even thought it is a long stretch, when ur infected, it is suppose to kill everything in your body, but from a mutation point of view o.k) His mother was bitten when he was in the womb and he was halfway infected coming out, I guess. But I have watched other movies where vampires say they where "born" fully that way, even thought it is all science fiction and fantasy, it is just now becoming contradicting and confusing. I can understand a werewolf because at the end of the day it is still alive, and can have babies. But if there is something I am missing with this vampire thing, someone please explain it to me, because I told my son he would have a better answer than which I can give him because my logic is getting in the way. Maybe there is a part to this vampire story I have not heard yet.