Whenever you want to tell people. It's not something that anyone can tell you to do. Some women want to wait until after 12 weeks, when the risk of miscarrying drops significantly. Others want the support of their family and friends, no matter what happens during their pregnancy.
For me, I always told people right away. I was way to excited to keep the news to myself! I wanted people rejoicing with me (and yes I've had 2 miscarriages out of 5 pregnancies).
I've told my friends and family already (I'll be 8 weeeks tomorrow) but I think I'll wait until I'm around 10 weeks since I'll have had 2 more ultrasounds by then before I let it be "common knowledge". Its so hard keeping it a secret though!
For ME personally....I prefer to wait to tell people until after the first trimester....my reasoning is that your risk for miscarriage goes way down...i went and told everyone about my 2nd pregnancy right after finding out only to miscarry at 9 wks. I may tell a few select people beforehand but anyone else like employers, co workers, acquantances etc. i wait.
I told the father (who told his sister) and my mother the day I found out, then i slowly told my sisters and my dad then we waited to tell family until i was 12 weeks, well it came a little before because my grandpas cancer kept getting worse (he passed away earlier this month) and i wanted him to know, hopefully for something to hold on for. We didn't tell Brandons family until 12 weeks however. I think its whenever you feel ready.