Oh, no!

Why not just make it that Ranma gets stuck in girl form, if she gets
pregnant?  Depending on the circumstances under which the pregnancy is
initiated, that could have some very interesting connotations.

Zen the Heretic-

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Hey that's a good idea... That could pave the way for Ranma thinking that 
he/she is permanently a girl, and at the end of your story, have him/her have 
a huge 'IM A GUY' party at the Dojo...

Of course, after nine months as a girl, he would probably be acting a bit more 
like one, so you get a good chance for some internal conflict (Ranma: Crap! 
I'm walking like I'm gay and there is NOTHING I can do about it! HELP!) 
hehe...