I would never even say such a thing to a woman even if she looked 8 months pregnant.
For example, the wife of one my cousins is a heavy woman. So many years ago, she was at my parents house, and standing next to another woman who I knew for a fact was pregnant. So I asked my cousin's wife when she was due because she looked at least 7 months pregnant. And she said "I already had my child"
Boy was I embarrassed. So as a rule, I would never ever ask a woman such a thing no matter WHAT they look like.