This was straight after a m/c and this time we told my parents and siblings at 5w in the knowledge that if it went wrong, I'd need their support - and we wanted to share the good news first, last time it was awful having to tell them the night before a D&C. We told DH's parents at 8w, after the 2nd early scan, and told DH's brother and sister at 10w, since it hadn't gone wrong yet... I'm now past 12w and we heard the hb on the doppler last week and again last night, but I'm still going to wait till the scan next week before announcing it generally. A few friends know because we were on a weekend away with them when I had a threatened m/c and had to go to A&E.
Having said that, one of those friends is due the same day as me and having had a scan at 7w, felt confident enough to tell everyone this week, at 12w, even though her scan isn't till the day after mine next week. I guess that's the difference between having a living child and an uncomplicated pregnancy, and having had a missed m/c and a threatened one!