People, if you're pregnant and you work, I advise you to tell them prior to your 21st week of pregnancy. The reasons why follow:
1) by 21 weeks you're likely obviously showing and they have to guess "is it a baby she's carrying or did she develop a tumor?"
2) you will be told they are now "behind" the information and planning for the future will be made more difficult by the lateness of the news sharing
3) you will be told it's not necessary to have all your "i's dotted and t's crossed" when sharing happy news with a family-friendly manager and that your manager is "hurt" that you didn't trust enough to believe she would be happy for you
4) potentially, your manager will not be the first to find out due to others in your department flat out asking if you're expecting and, at that point, what's the point in lying ... if you're manager is not the first to find out, she may likely be upset as mine is
In my defense, it was important to ME to have my "i's dotted and t's crossed" regarding my potential options as a part-time employee before I had any kind of coversation with my boss. Also, the last time I was pregnant I waited until I was 18 or 19 weeks pregnant before sharing so what's a few more weeks, really. And, finally, in my defense, I like my work/family balance and wanted to have my thoughts in order before sharing so that I could adequately convey my appreciation for my current professional identity. I guess there was really no way to win in this one, eh.