I'm sorry for all the rude comments people have endured. I actually was the rude comment giver, once. I didn't mean to be, though!
I was at a friend's work get-together (we'd just stopped by on the way to another activity). My friend left me standing talking to one of her co-workers. The woman was a bit overweight, but nothing extreme. She was talking to me about her baby and pregnancy, but not in a "my baby rolled over for the first time" way, more like, "when I found out I was pregnant, I was living overseas" kind of comments. In any event, she was kind of dominating the conversation and I was antsy to leave, so I'm afraid I wasn't really paying very close attention. (I know, rude right there!)
When she paused, I felt like I was supposed to say something, so I said, "And when are you due?"

Mistake. It got very, very cold there on the deck, and she said, "I'm not pregnant. I'm just fat."
Suddenly I was very much paying attention -- yikes! I backpedaled as fast as I could, trying desperately to act casual, "Oh, I didn't think so. You were just talking about your baby and I misunderstood." But still...OUCH.
That was probably 20 years ago and I still feel horrible!