This one is super for the butt and back of the thigh.
Get into an all-fours position. Keep the back flat with your tummy muscles held in tight. Bring the right knee in towards the tummy then push it out backwards and upwards, keeping the knee straight. Squeeze the butt nice and hard. Bring the knee in again.
Do 16 reps, rest and repeat.
Also see: How JLo lost ALL her pregnancy weight