whenever want. I personally feel earlier the better, if things dont turn out and your little one passes, then having people around you who know can then help you. if no one knew you were pg in the first place you haven't got the same support.
I told my family and bestie the day I got the positive. then didnt really announce it until I was showing (not til 20ish) only because I had already told the people who really mattered to me and who I really cared about. I did enjoy not having others (acquaintancees work colleges etc) know, was like my little secret. I told work at 16weeks only because I had to due to the type of patients we were getting and my not being allowed to care for them (chemo drugs and certain illnesses).
so long story should, people who I cared about I told straight away and everyone else I didnt