I usually say something rude right back. For example (not he same but it illustrates the point): my husband sexed me up pretty good one night and after I had to run to target for something. I was walking through the aisles with a goofy smile on my face and one lady leaned over to her friend and "whispered" "I thought you weren't supposed to drink while you're pregnant. Must be a horrible mom". When I caught up I her in the next aisle I casually leaned over and said "obviously you're husband can't show you a good time if you can't tell the difference between drunk and blissed out. my husband just f'ed me stupid" and walked away with my goofy smile. Needles to say she looked pretty Mortified.
When people say I look too big/small (it goes back and forth this time-no idea why) ill usually make a comment about how much they weigh or if they're calling me fat then bust out in fake tears. Nothing freaks people out more than a cryin pregnant woman.