I work with a lot of women. All of them who have had babies, in addition to myself, have this problem. It seems to be one of the battlescars of pregnancy -- irreversibly loosened connective tissue. This is true for fat & thin women, and all ages, though women who had their babies while thin and at a younger age seem to bounce back more effectively (since their skin was less stretched during pregnancy & the connective tissue less damaged).

You're at a good weight now. All of the medical resources recommend gaining only 15-25 pounds during your pregnancy. You can do this by watching what you eat & EXERCISING. Your body will bounce back much more quickly & minimal damage will be done if you exercise.

It depends on:
1) age & weight when woman became pregnant (young/healthy women have an advantage)
2) genetics -- some women can NEVER have washboard abs. Metabolism, fat distribution, etc... have a genetic component. Exercise can help all women to some degree, but some will not be able to remove the post-partum belly roll (contributed by loosened connective tissue) without surgery.
3) how complication-free the pregnancy goes (ie. some women have to deal with gestational diabetes or pre-eclampsia, and some have to stay on bedrest for several months!)
4) how much weight was gained during pregnancy
5) post-partum health -- some women can become hypothyroid or develop other permanent problems

Pregnancy is hard on a woman's body!!!!!