I was just about to post that comment Yvette!

That used to be my comeback for everything at high school. When I got called fat or a nerd, or stupid - whatever, my response was always yeah, but you're ugly, and I can "whatever fits here"

I did a search for witty comeback related to PG, didn't find much specific, and nothing for this situation, but I did find one I thought was a real gem, left here for all your enjoyment. Can't wait to use this one ! lol
"Actually, I'm not really pregnant, I'm a researcher and I'm doing a study about intolerance and stupidity in our society. Thanks so much for your contribution, it's been very helpful."