I do not believe they would allow you to do this if you are visibly pregnant. You have to sign a waiver at the beginning, but still, they would probably advise against. Now, if you aren't showing at that time, it should be a non-issue and only you can make a decision if you are comfortable with that risk.
BTW, when I did this tour, I did end up "crashing" my segway and I popped off. Luckily I didn't get hurt, but it is easy to do. Also, it takes some balance and your legs can get tired from staying in that position too long.
I don't think I would risk doing this tour while pregnant, and I generally not the most cautious person. But ultimately the decision is yours.
