Don't forget about Carl, was he born there too? We don't know his stroy yet, but he's about the same age as Alex. Maybe, the problem with births started after that. It may be related to people not having cancer. It sounds like people on the island have a super charged immune system, so they can be cured of cancer (except for Bens tumor, which isn't cancer) and none of the Losties ever seem to have problems you would expect in the tropics. Maybe in the D.I.'s effort to increase life span, they created the pregnacy problem