It is possible for some and not others. One of the problems people who have been very overweight and/or pregnant have is that their abdominal muscles seperate and thus having a truly flat, flat tummy is nearly impossible. Also, for those who ended up developing a pannus--- that doesn't go away on its own, even with substantial weight loss. Will it shrink? Yes. Will it disappear altogether? No.
But, just exercise, exercise, exercise and "diet" (eat healthier foods that are lower in calories than your body needs to maintain it's current weight) and you just might be able to achieve a good result without any surgery. Everyone is different.
Tummy Tuck
