When and how do you break your pregnancy news to your boss?

For my 1st pregnancy, I arranged a special meeting with her and tell her the good news. She was very happy to receive the news. Throughout the pregnancy, she is very caring and concern about me. After I gave birth to my baby, she even dropped by and visited me in hospital. For my 2nd pregnancy, I did the same but to a male boss. He was very surprised as I just joined the company for 3 months. However, I was very lucky to have two understanding bosses throughout my pregnancy as they did not require me to go to site visits after I pregnant. Personally, I feel it is good to let your boss to know about your pregnancy as soon as you find out. This is to prevent any misunderstanding e.g. boss complains about your work performance since he did not know about your pregnancy and etc.