Ok, so you had me curious and I googled

Here are some movies that you can pick characters from

Father of the Bride 2 (Diane Keaton - bit old, but her "daugther" is also pg in this one)
Nine Months
Baby Mama (those two chicks from Saturday Night Live)
Knocked Up (she has some classy outfits in this one)
Rosemary's Baby - a bit eeek, because she is pg with Satan's child, but it's an "old" movie, so the costume might be good.