Your boss does not need to know how soon you found out, you could always say you wanted to wait til past 12 weeks in case!?
That's your choice & no they would not be already looking for anyone yet anyways!!!

I'd just say I just found out & I am about 10-12 wks & will let you know then once all is OK I'd then tell them... Don't worry at the end of the day it's just a job & unfortunately we are all replacable!!!!