After you have your baby, your belly will slowly go down on its own. However, you can purchase the exercise mat from the built in store on the hud to help your belly go down much faster.
The mat is equipped with several exercise options for you to choose from. When you click on one, your avatar will start animating that exercise and you will get a message that you are feeling thinner!
Now my belly is nice and flat again, back to my pre-pregnant shape.