From a side point of view:
A. If it's a new Mum check with the Dr/Physio to make sure it's OK to exercise etc, as far as I know some Mothers move the stomach muscles when pregnant so need special treatment etc.
B. Once given the OK from the Dr etc, then any exercise that is gentle. I would suggest Yoga, and Swimming, nothing too vigorous. I guess the post birth hormones may effect the muscles so some care is needed.
Lastly, as said above once the all clear is given, do what you enjoy, whether it's walking, riding, biking, lifting weights, what ever. It matters not!