We told all the parents when we found out (5 weeks) with my first pregnancy. My mum decided to tell her friends, some of whom I know, against my express wishes. So when I miscarried at 7 weeks, I was receiving congratulatory emails from them when I was in the midst of everything. It wasn't fun.
This time, I had to tell my mum after we found out (4 weeks) as she was staying with us and it was really hard to diguise that morning sickness. She still went and bloody told people (I had to have stern words with her, she was still baffled). We told everyone else after the 12 week scan, even though it was at 11+2, since we wouldn't be having another scan until 20 weeks and we didn't see any point in waiting the 5 days to 12 weeks, IYSWIM.
We told parents in person and everyone else via the whole Facebook scan photo thing - our friends and family are distributed all over the world, so it was easiest this way.
Only tell people you can trust to keep their mouths shut if you don't want it to be common knowledge, that's my advice. I did feel a lot more confident in this pregnancy after the 12 week scan, so felt more comfortable telling people. It still feels like a hell of a risky thing, though - I almost feel responsible for 'keeping' this pregnancy as other people would be disappointed, upset or look at me funny if it goes wrong again. I don't really do public displays of emotion 