When should I start letting people at work know?

Sha
I'm not due until early August but I need advice on when I should start letting people at work know. Since I am only 5 weeks along I'm not showing yet, we plan on telling family (which is another story in itself) and close friends around the 12th weeks but as far as my job that's the question. I'm up for a promotion soon and I don't want my pregnancy announcement to get me over looked for the promotion nor do I want people treating me like I'm helpless around the office. What do you guys think I should do?