As I am now "showing" let me share some good or bad ways of asking or noting that a woman is pregnant.
1 - A colleague/friend (knowing I'm pregnant and teasing me) - "Whoa, are you pregnant or just fat?" I laughed. It was especially funny given the mortified expressions of the attorneys around us. N.B. most women won't like this question.
2 - "Whoa! Are you having twins?" Less funny, as it was stated in all seriousness by someone in church.
3 - "Are you having twins?" Uttered a few minutes after #2, by someone I know even less well, again in church.
Might I suggest it is perhaps not a good idea to ask someone if they are having twins. If you think they are huge and wonder if they are having twins, feel free to ask something like "so, what are you having?" or perhaps "how far along are you?" This will allow the large (hopefully pregnant) lady to state that she has been blessed with twins and is therefore really big. Also, it won't make the pregnant woman who is only carrying one child want to punch you.
For the record, no we are not having twins (unless one is really really really good at hiding) and I have only gained 11 pounds, it's just all in front.
I learned to never ask when I was switching law firms as a 4th year associate. While I hadn't started at the new firm yet, I was invited to their "prom" [you can tell how long ago this was by the fact that firms even had proms]. I was near the library [still a PC term then], and one of the woman had on an elegant black lace cocktail dress shaped like a flared tent. I asked her when she was due. Such a look of dismay crossed her face as she said, Oh, you must think I'm pregnant". My attempted save was, "No, you just look young enough to be." - not a good attempt at all, since she was probably *maybe *in her late 30s or early 40s.