My Doc (before ttc) told me that horse riding when pg is fine as I already ride regularly. Concern is that you will do something that your body is not used to. In fact it is my understanding that it is important that you keep doing what you normally do while you can to prevent loss of muscle and fitness.. just don't take up new sports or put yourself in way of harm.
As a horse rider that means no longer riding 'unsafe' horses, sticking to the ones that I know wont cause me too much grief. ie the old and faithfuls.
Means I can keep throwing hay bales, chasing cows and riding horses until my body gets too big and I can't move or I feel the situation is unsafe or too risky.
Means that the planned aerobics sessions are now out for me as I haven't been doing them recently and now is not a good time to recommence.

Joy.