I have a friend who I know is trying to get pregnant. A few days ago we were having lunch and she said she had been so exhausted ever since she got back from a conference out of town. She attributed the exhaustion to having prepared hard for her presentation, and all the anxiety that led up to the meeting. So I controlled myself and didn't ask the obvious.
Most people try to keep their pregnancies private at first. It is probably a good idea not to tell casual acquaintances that you are expecting, especially since one in eight pregnancies ends in miscarriage. It doesn't make a loss easier when time and again everyone you told asks how the pregnancy is going. So one rule of thumb is not to tell anyone with whom you wouldn't want to share bad news.
But keeping mum is motivated by more than that, because people often don't tell their parents or closest friends about an early pregnancy, even if they would disclose a complication like miscarriage. I don't know if it is just not wanting to celebrate too early (like it would be bad luck) or perhaps wanting a little bit of privacy before your news becomes public information, a moment to hold to yourself before the world changes.
My friend probably isn't even expecting. Obstetricians always think everyone is pregnant. I am always the first one at a movie to pick up on the cues--like the main character can't quite button her pants, or is gaggy in the morning. At work, all it takes is for one of our female residents to get a stomach flu, and fifteen people come running at her with pregnancy tests.
And if you are in a certain demographic, don't even consider not having a glass of wine when offered at a party. It is like a birth announcement. That is why I suggested in The Working Woman's Pregnancy Book to carry around a drink that looks like alcohol when you are still in the non-disclosure mode. You don't owe anyone your personal information. And taking my own advice, I am going to continue to try not to ask. We'll see how long I last.