As my last pregnancy ended in an early miscarriage I'm very wary about telling family and friends too soon. Ideally I would like to wait until maybe 10 - 12 weeks, but I very much doubt I'll be able to hide it for that long. My MIL seems to have pregnancy sense so she'll no doubt figure it out pretty quickly. In reality I'll probably end up telling family after my first midwife appointment, at which point I'll be about 9 weeks.
I have told a few people already. I told my sister because I trust her and tell her everything, and I have a few mum friends I meet up with frequently whom I've told as I know they wouldn't mention it to anyone else. The reason I've told these people is partly so I have someone to talk to in real life who have been there, and also if I do lose it again I'll have support.
I won't announce it to anyone else outside of family until my scan when I'll be about 12 weeks. I'm also going to be a bit cheesy when announcing to family and stick my daughter in a big sister t-shirt.