I know everyone said don't tell...so here's a quick story.
I found a job when I was five months pregnant with my first that I was a great fit for and ABSOLUTELY qualified for. In the preinterview over the phone, I basically had the job with full time hours and benefits.
During the interview, I let it be known that I was pregnant and that I didn't plan for it to interfere with work...and then, all of a sudden, it was "Oh, well, it's just an on call position." Instead of the full-time, great paying job I thought I had.
I was so shocked that I flat out refused to take it, and you know what? I never did go back to work. Six years later, I am still at home and loving it, now with a second baby.
That being said, I STILL think you ought to tell...I think it's only fair to let an employer know because the reality is, it DOES affect your job...and I know they shouldn't discriminate, but a good employer WON'T discriminate. And if they DO, it's probably a job you don't want. Plus, a good employer will appreciate your honestly, and the same good employer will not like to be misled.
But, that's just my opinion. If you want to be guaranteed the job, don't tell...if you want the job to start out on the best foot, tell them with the risk of not getting a job (and then be happy knowing you wouldn't have liked people who discriminate, because you'll still have to face days at home with sick kids, etc.)