Think of a rubber band stretched too far.

[tarakul islam]

The worst of cosmetic nightmares is scarring, and stretch marks are
scars much too easy to come by. They are tears in the dermis, the
flexible middle stratum that maintains the shape of the skin. The
tearing is caused by the pulling that occurs with accelerated growth.
The rapid transformations of puberty or pregnancy are common
aggravators. So are weight gain, muscle-building, and the uses of
certain steroids.
http://stretchmarkinstitute.com/