Ok I was wondering about this too and you guys seem to ahve come up with the best explanation - that if you concieve pre-island then your baby is fine. This was kinda hinted at by Juliet in flashback, and it seems she was right. As for Ben, he coulda been born outside the island (to avoid him and his mother's death) and moved there very young.

As for the calves and the baby boar and all other wildlife on the island?? Maybe it's a human specific condition :S