A.C.
How awful. What yahoos. I never ask a woman about pregnancy unless it is VERY clear (we're talking 9 months and clearly in a maternity top).
I'm sorry that happens to you, but I would suggest you try not to be rude back. Just as a rule that's the best way to be (you know, "turn the other cheek") and it really does make you feel better about yourself if you come out as the polite one. I would just say something like, "I had him 10 months ago," and then change the subject with something like, "Here he is, isn't he a cutie?" Trust me, they will feel stupid enough.
Just to maybe make you feel a little better, though, remember, they are probably asking because they want to acknowledge what they mistakenly assume is your impending joy and congratulate you. Even though they should do a better job of thinking before speaking, they are trying to be nice, not mean.
Try not to feel too bad. It happens to a lot of people. When I was in my mid-twenties and a very skinny size 4, one of my high school students asked me when I was due as well! It is natural for a woman's stomach to be round. Those six-pack abs, on the other hand, are NOT the way God made us!
~A.