The other thing I would say is just plain comfort. With pregnancy beyond a few weeks women often report that their joints are laxer and that they can get out of alignment easier. With the jolts and shakes of a roller coaster, it may mean pain for pregnant folks. I don't think that roller coaster riding, and the physical demands on the body, have been well-studied in pregnancy, if at all. All I can say is that it's a man-made phenomena and I'm not sure that the amazing design of the pregnant body was made to withstand the unique pressures! Saying that, like most things, moderation is the key, probably.