I was five months pregnant on one trip and two weeks pregnant on another (didn't find out until I got home!). I think most rides you should stay off of are marked with warnings (Splash Mountain, BTMR, Space Mountain, RRC, TOT, Body Wars and Star Wars). I would also not do any water slides, even the ones with inner tubes. I went on one when I was two weeks along and I was bounced back and forth and up and down so much that I was worried then!
I've heard that people have gone on some rides at your stage of pregnancy, but I figured I would rather sit out and not risk any problems. A few rides are definitely not worth causing a problem to the baby. The great thing about Disney, though, is there is so much to enjoy without going on the "not-for-pregnant-person" rides.
Remember to drink lots of water and put your feet up often.
Have fun!