Use your pregnancy to develop a character for your homemade Halloween costume. Lots of characters aren't funny by themselves, but can be very funny if you add pregnancy to them. Try being a pregnant plumber, with low-hanging jeans, a work shirt, a baseball hat and a fake mustache made out of construction paper. Or be a pregnant doctor, nurse or dentist, using scrubs and the appropriate details, like stethoscopes and face masks. Try a pregnant football player or other sports character.
Use your pregnant belly by making a homemade costume that incorporate a "beer belly" or gut, traditionally associated with rednecks. Cross dress and become a hillbilly with a huge beer gut, straw hat and bandanna.
Use your belly as an object in your costume. For instance, if it is warm where you live or where you are going, paint your belly orange, and wear basketball shorts and a shirt so you look like a basketball player holding a basketball. Make your belly into a fishbowl, and be a person holding a fish bowl. Make a bird costume by wearing a sweatsuit and gluing or sewing feathers all over it, and paint your stomach white so you appear to be holding an egg.