Immediately postpartum you'll probably feel pretty skinny! The first time you stand up and you realize that your belly is now a gelatinous mass...well, I'm sure you get the picture. It took you nine months to get pregnant, and it will take you awhile to lose the weight and tone the muscle again. Talk to your practitioner about safe exercises in the postpartum period and about checking for separation of the muscles (diastasis recti) and how to deal with an abdominal scar, if you had a c-section.
How quickly you return to your prepregnancy size and shape will depend on weight gain, whether you had a cesarean section or not and many other factors. A healthy diet and exercise are very helpful towards these goals. Though many moms note that even after they have lost all of their weight they still have a different shape than they had prior to babies.
You will also want to take into consideration that even if you follow a diet, exercise daily and do everything exactly right, you may still never have your abdomen looking exactly like it did before you had a baby. In fact, it is highly likely that your belly will stay different. But remember, this doesn't have to be an unhealthy difference. Talk to your practitioners about the healthiest ways to get back in shape after having baby and even what to do about unwanted stretch marks after baby.