When to tell the boss im up the duff?

i have just found out i am 12 weeks it was unplanned i have been on the pill (and yes i take it same time every day) so needless to say i am a little shocked. my partner and i have decided that we are not going to tell people until we have to, basically when i cant get away with saying i have put on a little weight. no getting to the point my question is when do i have to legally tell my boss who i have been with for 18 months that i am up the duff