Coming out of the pregnancy closet

This is a weird one. It is purely a private and personal decision when to tell people you are pregnant. Some people tell straight away, some wait till "the safe week" and some wait till they start showing (well, that would be around now then).
Twice, i've had people at work asking me if i'm pregnant. It's a bit like asking people if you're gay. What do you say? Are you gay? Yes, but i'm not ready to tell anyone yet, so please keep it to yourself (yeah right); That's a personal question, you shouldn't ask people that (which still means yes); or... Lie and say No which makes you feel sad and mean to your inner gay (or baby) because you're so happy that it's there and you don't want to deny its existence.
So people just shouldn't ask. Why do they? You wouldn't ask someone if they were gay unless you were a really close friend or their mother. In which case you would probably confess all, unless they already knew of course.
I told a friend (who knows) and she said it is kind of written all over my face. I look really tired apparently, but i might have been ill or something. But no, apparently this is something other women can just tell. A bit like having a gaydar.