It will depend on your situation, if you see family all the time,i am guessing a nosy MIL will be looking for signs, and if you are unfortunatle to get really sick, she will pick up on it!!!

I told my bosses first actually, as i have had trouble carrying a pregnancy through, i felt that they needed to know right away in case anything bad happened. And also cause my DH banned me from doing any physical work when i got pregnant, so like fiona said, i didnt want to make bogus excuses for not helping out in the bar and kitchen etc (i work in a hotel, mainly book work and office work, but i was usually available to help out in the bar etc if it was busy, not when pregnant though- so i had to tell) but i kept it from family except my mum til the 12 weeks scan. Then i just made a few phone calls, and let them all spread the word around them selves.

Good luck TTC!!