There's actually a great DVD out called preggie bellies that I bought at Rebel Sports. It's good because it uses a fit ball and weights (which you already have!) and shows good workouts for women at different fitness levels. I thought it was great in my first and second trimesters, and have used it a little in the third...mainly arms though, as the bouncing on the ball can be really tiring.

Hope this helps!

Julesy