Ah, that situation! Congrats on the job! I got a new job (moved back to the country and needed health benefits ASAP) when I was 13 weeks pregnant with #1 (thank god my suit still fit) and then hid it for another, oh, 5 or six weeks (buying bigger pants, wearing sweaters, it was winter
It went really well when I did tell them, so I hope it goes well for you as well! And remember, pregnancy is NOT a condition you can be fired for!