belt doesn't prevent your tummy from looking big. it is the weakness of the muscles which leads to the looseness and makes your tummy look big. you need to get your abdominal muscles intro shape. for that you need to exercise. you could contact your Gynecologist for guidance or a physiotherapist.

belt does not reduce tummy, it only gives support to tummy. to reduce tummy, start exercising. start with walking. you can do cardio, yoga etc. after 6 months... (actually yoga can be started next month after deliver, but only under guidance of an experienced yoga guru and clearance from doctor)