If I were Ranma, I would immediately use the second wish to wish her not-
pregnant (and even never-was-pregnant).  Pregnancy is about the only event that
can _really_ screw things up _permanently_ for Ranma short of major characters
in the series dying.

I would expect him to word the wish well enough not to get stupid results like
just undoing the first wish, because it's not an urgent "I have 10 seconds to
phrase this wish" sort of thing.

Of course, if she's lying, Ranma can make the wish that she was never pregnant,
then find out there's no change, and then find out he has no way to explain
to everyone else how he knows she's faking....