People who ask "are you pregnant?"

Why?

Quick story: I was in an elevator with a coworker I didn't really know and he told me a story of when he asked a stranger in a restaurant if she were pregnant. She was not! And he said he was so embarrassed that he had to leave.

I didn't get a chance to ask him, so I'm asking you fellow tilderinos - why ask this question at all? Especially to a stranger? What motivates this question? Is it really asking why someone looks fat?

Have you been on the receiving end of this question? (If you're a women older than 25, I'm going to guess yes). What are your stories?