I meant to wait until late in the first trimester, but my parents guessed way before that. (Didn't have anything to do with me being grumpy and sleeping all the time, I'm sure!
) They pretty much spread the word to the rest of the family.
I waited until I was about 10 weeks to tell my boss and people at work, but since I'm in grad school and my assistantship will be over before I deliver, I could have waited until whenever. I don't know what I would do in your case!