New job

I don't know when the right time to tell them I am pregnant. I'm not showing and I feel fine. I know it's illegal to discriminate against pregnant woman, but I feel bad for not saying it at my interview. It's a desk job at a busy office, I know I can handle it and will return back to work as soon as possible after having my baby since my mother will be watching my baby while I work. I just don't know when to tell them. I'm a bigger girl and i haven't gained any weight and I'm not showing. I was thinking when I  start to show.  Any help would be greatly appreciated.