The last time I went, I was in my second trimester. I actually had a chat about this with one of the castmembers. I was disappointed, because I didn't get to ride on anything good. I was talking to a SM castmember, and they were telling me they see pregnant people go on roller coasters all the time. They said they wouldn't stop me, but they would discourage it.
Of course I didn't do any roller coasters to be on the safe side, but I did Autotopia and Indiana Jones. I kind of held onto the bar on the IJ ride so that I wouldn't be thrown around. It was definitely okay. I actually did not mean to go on that ride. I was in line for 40 minutes or so, and it wasn't until I reached the head of the line that I saw a sign. The CMs there told me I would be okay, so I went ahead and did it. I had been on it in the past, and had an idea of what it was like.
I forgot to mention, that there are signs that let you know whether or not you should ride while pregnant. It's up there w/ the heart failure and elderly.