I told my unit leaders when I was only a few weeks preggers in case I had to dart out of the room suddenly (one of the leaders shared a double portable classroom with me). However, I told the rest of the staff when I was 10 weeks because I was sick of them asking what was wrong and staying away from me because I looked sooooooo crook. I asked the Principal to announce it at a meeting so they all found out at the same time. It was a weight off my shoulders!
Hopefully next time I will be able to wait until 12 weeks before telling people just because it seems to make the pregnancy drag for everyone when you tell them early, LOL