lol i have no idea, but i think its called suspention of disbelief :D

Stephenie Meyers explains it on her website.
Here's the link: link

It's under: Vampires and pregnancy: when did that idea occur to you? How does that work?

um well a male body will never change like a vampire can still make sperm even if hes like stopped growing just a girl vampire cant make a baby cuzz there bodys will have to change in order to make the baby but Bella isent a vampire when she gets pregnant so her body can still change so there you go.

thanks guys