Henry - we told parents at 9 weeks with my first, close friends at 12 weeks, and the rest of the world found out on their own at or after 18 weeks - when I couldn't hide it anymore. With #2, we told family and close friends after our first good u/s - at 5 weeks - cause we wanted as many prayers as possible. We told other friends around 7-8 weeks and the rest of the world at 10 weeks. I felt very confident about the 2nd pregnancy from the very start, so I wasn't worried about telling people, and we wanted as many people praying for us as possible. I was totally freaked out with my first pregnancy, so telling anyone at any point scared me.
Just do what feels good to you. Don't feel pressured to tell anyone, and don't hold back if you want people to know.