One I like was making the belly big on the girls. So you can have one look pregnant to add more to the RP.
Did that with an NPC in "Bait and Switch". Romulan with a half-Bajoran baby (it makes sense in context). It actually doesn't look half-bad if you also increase the bust size a bit, which happens during human pregnancy anyway. I wouldn't try to use it on a boff, though: A, when they're that pregnant Starfleet would most likely send them on maternity leave, and B, the char animations can't really do the right bounce.