I started a new job when I was only a few weeks pregnant, and left when I was about 26 weeks pregnant, didn't tell them a thing. But in my case I knew Id be leaving before I was due and I was actually taking it all with a grain of salt.

I think though that the best you can do is say straight out and honestly that you are serious about this job, your pregnant and are having a very low risk pregnancy (you are aren't you?) and that you don't see that that should get in the way when you intend to work right to the end and return to work as soon as possible.
That way your telling them outright, and that you are serious about this job and most of all your being honest.

I dont really know what else to do. If you really wanted to not mention it and then tell them later then I think that would still be legal, but one cant be sure as to how they will take it, like you said.