I didn't tell anyone until 13 weeks, when the first trimester ended. My sister had 5 miscarriages, and I had friends who have also had them. I was so scared of telling people and losing the baby.
I saw my OB the week after we got a positive. I was right at 5 weeks pregnant. The office I go to, doesn't have an 8 week policy. As I understand it, most do. I had a higher risk pregnancy, so I was getting high level ultrasounds every few weeks. At the end of my pregnancy, I was in my regular OB office every week getting an ultrasound. From my OB said, most people have 3-5 ultrasounds during their pregnancy.
ETA: I actually told my work right away, because I was SO sick from day 1. I actually lost my job 2 days after I told them. (TOTALLY unrelated reasons. Many of us were laid off.)