It depends on your fitness level, I used to exercise a lot....I started to exercise after week 13....aftr I got my energy back...include yoga, pilates, walking, stationary bike, light weight training.....
not only for keep fit...exercise can strengthen the heart because we are pumping blood for ourselves and baby, give more oxygen to baby, it helps us when we give birth and keep strength when we need to take care of new born baby
I'm week 33 now and still exercising, with not much back pain