Though the story does have some real elements..(the name of the towns and cities..tv shows...cars)..its still just a story. Its not real (no matter how many us which it was!!)...its a world created দ্বারা Stephenie Meyer (bless her)...anything can happen in a fictional story.
Ok since Bella is human her body can change to carry a fetus, rosalie, alice and esme can't since they are ভ্যাম্পায়ার thier bodies can't ever change again. edward is male so he nor his brothers are not affected as harshly they can't carry kids anyway.
The only logical answer i can come up with is: THEY HAD SEX. It doesn't really make sense to me either though, because doesn't a man have to ejac so the sperm can fertilize the egg? If Edward is ফ্রোজেন in time, how can he still ejac to get Bella pregnant? It's fiction, that's all i can say.