That's a good rule. As much as I once liked to read this kind of stuff (if the universe it happens in at least supports a bit the idea, i.e. very, very, very sci-fi or heavy magical environment), it can get pretty silly when done in a rpg. I have seen it happen. In an Transformers rpg. Between two bots, even though it is canon that they do not procreate in such a fashion. It was very... enlightening.
So, mucho danke for this rule.
Oh, and a question (as we're talking about pregnant stuff). How is the rule about inter-species pregnancies? Is it ala Star Trek or something other?
(I don't know why I even ask this, Jazz is definitely not genetically compatible with any character in the game.)