Stretch marks are a common problem after pregnancy. The result from actual tears in the deeper part of the skin when the tummy rapidly expands, causing small scar to actually form under the skin's surface.
There are a few treatment options for these, but none is better than a tummy tuck. A tummy tuck actually removes all of the skin from the belly button to the pubic hairline (it also tightens the muscles with an internal "corset," see the photo below!). The remaining skin from the upper abdomen is then pulled down, providing healthy new skin over the entire abdomen.
A tummy tuck results in a scar, which usually hides inside the bikini area (if your plastic surgeon is a good one and plans it right!). You can also get a tattoo to cover up the scar. Women tend to wear lower and lower bikinis these days (that makes me sound very old, doesn't it) so this may be an option as well. This link on pinterest shows some examples of tummy tuck tattoos used to camouflage the scar.