I work in a fairly large office where people always seem to be talking about who's pregnant or who wants to be and the general benefits of avoiding work and being paid for it.
On occasion, I've heard people make the rather large mistake of asking "When are you due?!" to someone who has to tell them they're not pregnant and then it's horror all around.
I'm terrible at guessing games and so, avoid asking questions at all. I find it much much more direct to say "Wow, you're really packing on the weight!" and keep moving.