All of the attractions will have a sign that states whether or not expectant mothers, among other people with certain ailments, should ride. For the CYA answer follow the signs and the advice of your OB.
Personally I rode everything I wanted to and that included mulitple rides on ToT, RnR and all of the thrill rides. Granted I didn't know I was pregnant at the time and didn't find out until we were back from our trip so I can't say 100% that I would have done all of the attractions had I known. I asked my OB about any possible damage/scrambling and she assured me that our future family addition was safe since he was so tiny at that time and pretty well protected. Eventually our oldest was born healthy and very vocal.