If you want to get really creative you can make your own costumes, here are a few costumes that you can make just using your belly and normal clothes are:
The eyeball- This is something my husband created, all you need is a little facial paint and black clothes. This is something even a person that can't draw can have a little bit of fun with. Wear all black clothes on Halloween and paint your belly white, make the center of the eye whatever color you desire and don't forget the red veins. Make it scary or cute, whatever fits your desire. Have a bit of fun and show off that tummy by keeping the shirt up or wear a shirt that shows your tummy. My husband got a kick out of this one and was so excited that he got to do the artwork.
If you want to dress up completely but not stand out, try going as a human pumpkin. Wear a tiny green cap or make one out of felt and all orange, if you can cut a slit in your shirt for your belly to stick out. Now, paint it orange and decorate your pumpkin.
A pea is another simple costume you can make and stay warm. Go find some green jeans or sweats and a green sweatshirt. Keep the shirt up or cut a hole in it to make your little pod stick out and paint it all green.
If you aren't feeling up to having a full body costume, you could always just paint the belly and proudly show it off. After all, how often will you be pregnant? Think of anything that is round: basketball, volleyball, soccer ball, a bowling ball, etc
Don't forget witches were mothers too, so if you've always dressed as a witch for Halloween you don't have to stop the tradition. This year go as a pregnant witch or maybe a witch holding a pumpkin. Use that belly!
A Bun in the oven- Get a medium or small box, cut it out so that you can wear it over your shoulders. Make room for your head to slide through. Decorate it with paints like an oven. Cut open the front of the box so that it will open and close. On the outside of the box write, Bun in the Oven. Paint your belly a light beige for all to see the little baby bun.