legally you dont HAVE to tell your boss till 10 weeks before you are due to go on leave. (which is generally 6 weeks before the baby is due - many companies ask you to leave then for insurance purposes)

I guess it depends on your situation - I told mine the day after I found out because she knew I had been through 2 previous mc's and I needed her to knwo I would be a bit "funny" for a while.... she was very supportive.

So, for now, up to you my dear!!!

Congrats!