I just got back from a trip to Disney World (at 23-24 weeks), and I did sit out all the big thrillers. I have absolutely no idea if this is true, but on the Disboards it is frequently posted for pregnant women that Splash Mt. should be avoided because the angle of the drop mixed with the sudden stop at the end can lead to placental abruption. Like I said, I don't know if that is true, but even the idea of it was enough to get me to avoid the ride. There is also frequent warnings that the roller coasters should be avoided due to the g-forces.
I didn't ride on Dinosaur at Animal Kingdom, and it is supposedly the exact same ride as Indiana Jones at Disneyland, only with a different theme. DH rode it and was really surprised at how bumpy and jerky it was. I was glad that I had skipped it.
The only ride that I rode on my trip that had the expectant mother avoidance warning was the safari at Animal Kingdom. I had ridden it plenty of times before and knew that it was basically just a bumpy truck ride, so I didn't have any concerns.
The good thing about Disney World and Disneyland is that there are so many fun things to do besides thrill rides.
This isn't what you asked about, but I will say that the biggest problem I had was with my feet swelling. They swelled up so badly by the end of the first day that none of my shoes fit, and I had to buy a pair of Crocs in my resort gift shop so I had something to wear on my feet. That was not fun. So, if you are going on a multi-day trip, you might want to make sure to put your feet up every day. (I hadn't had any problem with swelling before the trip, and I haven't had any problem since I got back.)