Never ask a woman about her pregnancy unless she brings it up first.
I been present a couple of times in a group of casual conversation when other people (thank God not me) have asked a woman who appeared pregnant when she was 'due'... and one time the woman was just overweight, and another time the woman has just lost a 6 month stillborn fetus a week before and the husband had to answer for her since she just couldn't speak. That was really bad. A good lesson in discretion.