I had an m/c 2 years ago at 7-8 wks and when I, very happily, became pregnant again, I did tell my overall boss at 8 wks just in case everything went wrong and I would need time off again; I do not like my immediate boss and felt that it was none of his business, but if there was a problem, I could just call in and tell the overall boss.
I did tell some other work colleagues around wk 11 as was starting to feel more hopeful; also as they knew me, the m/c and my plans to start IVF (I ended up getting pregnant naturally), I told a few people as I knew they would be delighted (they were), but my immediate boss started to be a jerk about antenatal appointments, so I went again to the overall boss with Equality Authority information, etc., etc.
I'm now almost 18wks and cautiously hopeful (am 43 and everything so far, looks good)!
best wishes
p.s. my immediate supervisor had me moving classrooms, going up and down three/four flights and told me I couldn't go to the bathroom during class -- I told the overall boss and magically, my classroom was moved, bathroom breaks ok, etc., SO only if you feel you are doing too much, then mention to one or two people...also, I'm 43 and in good health, my pregnancy seems to be 'normal', e.g. tiredness and the usual stuff, but I still get people to carry heavy stuff, etc. Better to be safe!