I agree that it depends on your situation and the type of work you do. My job is very physical so I had to say earlier than I would have otherwise...
I had no choice but to tell my boss at 9 weeks. I have HG so i have been really sick since day 1. I needed her to know why i was spending so much time in the bathroom and missing work for so many dr apts and all that
I think it depends. Into the 2nd trimester seems good. Maybe even a little later. You just want to give them enough notice to accommodate your maternity leave. I'd tell them earlier if you're having a difficult pregnancy or do more physically demanding work.
I'm 30 weeks and haven't told them Lol...I wasn't showing much until these past two weeks but I'm telling them next week...I've just put it off because I transferred from a different location (same company) different positions so I had to start probationary period again and I'm nervous what will happen but I have to tell them now. Good luck!