I actually was in that situation too and i did not tell my potential boss at the interview - however once i had been working there a few weeks i told her that ihad just found out ( i was about 11 weeks) and she was really cool with it and when it came time to have bub i was given as much time as i wanted off and she covered me with temps - I took 3 months off and then went back and expressed etc I was working only 1 day a week prior to bub and now do 2-3 and i asked my boss the other day why she was so flexible for me and she said that she probably wouldnt have given me the job if i had come announcing my pregnancy but once she had me there and knew how good i was at my job , she was willing to so whatever to keep me. (I should have asked for a pay rise then!)

Good luck and jut do what feels right to you at the time - i HATE lying but really wanted the job!