There is a sign.
That said, I rode it 7 months pregnant, sitting somewhere in the middle-ish. My 9 month old fell asleep on the ride. (my younger two kids are 11 months apart). We had ridden the ride several times before on previous non pregnany trips, so I knew what I was getting in to.
We rode again the following year when Iw as not pregnant, sitting in the very back and it was massively bumpy, erratic and I was actually afraid my kids would bump out! So a lot has to do with your seating in the jeep and the driver.