I agree with Ivana. It is fairly easy and you are stopping and starting all time. The worst bit for me was the climb up over the arch. My legs were killing me when i was clinmbing and to stop made it worse, but once at the top of the arch it is fine. It was kind of like being on a stair climber at the gym but the steps are a bigger deeper step forward than a regular step IYKWIM.

Not to worry you though but i am sure on the liability thing you sign before you start it goes through medical history etc, and i am sure it asked if you were pg and do not allow you to climb ( if you ring them before hand they will be able to tell you). I suppose that is to cover thier buts in case of anything happening. At 12 weeks i don't imagine that it is obvious you are pg, so go with your gut instinct. If you think you are fit enough and your health has been good for the pg-good luck and enjoy the view.