It is totally a personal decision. But, if you decide to tell people before the first trimester ends (at 12 weeks your risk of miscarriage diminishes significantly), you should be prepared to tell them if you miscarry. The first time I was pregnant I told family and close friends as soon as I found out, and almost everyone else at 11 weeks ( I was starting to show and I thought everything was fine). Then, 2 days later I miscarried. So, I had to explain to everyone who hadn't heard I miscarried, for a few weeks after it happened (word travels fast). So, when I got pregnant again, I told family and close friends as soon as I found out, and waited to tell others until 12 weeks (I had an ultrasound at 7 weeks and heard the heartbeat again at 11 weeks). I believe that if you do miscarry, you want close family and friends to be there for you and help you through that rough time. So, I told them right away. Co-workers pretty much guessed I was pregnant again (morning sickness and I was showing at about 9 weeks), but I didn't confirm/tell them until 12 weeks. I just didn't want to go through the pain again of having to tell people I miscarried, as it is such a personal thing. I took it for granted that it wouldn't happen, but it happens more than you think.