People who are considering the implications of introducing pregnancy into Fable might be interested to know that a player made mod for Morrowind exists which actually allows for marriage and pregnancy. You can read about it
here.
I hope this is not too off topic, since it deals with another game entirely. (my apologies if so) But, given the similarity of subject and issues, it seemed reasonable to post the link.
I'm still not sure about the whole idea.... As I mentioned above, it really would depend on the implementation and NPC development.