Stretch marks, also known as striae, are a form of scarring on the skin with an off-color hue. Over time they may diminish, but will not disappear completely. Stretch marks form during rapid growth of the body, such as during puberty or pregnancy. In pregnancy they usually form during the last trimester, and usually on the belly, but also commonly occur on the breasts, thighs, hips, lower back and buttocks. These are known as striae gravidarum.
Stretch marks are caused by tearing of the dermis. This is often from the rapid stretching of the skin associated with rapid growth or rapid weight changes. Stretch marks may also be influenced by hormonal changes associated with puberty, pregnancy, bodybuilding, or hormone replacement therapy.