Luckily, I've been spared being asked the pregnancy question, though I'm surprised I haven't been. I'm carrying more than a few extra pounds and I'm busty, so if I wear the wrong style of shirt or dress, I can easily look preggers.
However, an old manager of mine did get the 'when are you due' question. She had had her son about 8 or 9 months before and she was starting to wear some of her pre-pregnancy clothes again. She was sitting in a meeting where she knew a few people, but not everyone. During a break in the meeting, she said the girl next to her turned to her and started chatting then said something along the lines of "Oh, I love your shirt, it's so cute. When are you due?" My manager said that she was stunned speechless for a second, then said "I'm not." She said that the girl just said "Oh," then kept talking as if nothing had happened.
Let me clarify that the shirt in question was indeed cute, but in no way resembled a maternity blouse. The fabric was a bit clingy though, and it didn't hide the fact that my manager hadn't lost all of her "baby belly" yet.
I know that the girl in the meeting was just trying to be nice and friendly. But I don't think she has any idea how badly that comment hurt my manager's feelings and self-esteem. She said that she pretty much wanted to burst out in tears, but seeing as how she was stuck in a meeting for another few hours, she had to keep her calm. I felt terrible for her. I don't think I ever saw her wear that shirt again unless she had a blazer or sweater over it...