Stretch marks are a form of scarring on the surface of the skin with a silvery white hue. Stretch marks are the result of the rapid stretching of the skin associated with rapid growth due to weight gain or pregnancy. They are caused by tearing of the dermis, and over time can diminish but not disappear completely. In dermatology parlance they are called striae.

Although the skin is fairly elastic, rapid stretching of the skin will leave permanent stretch marks. Stretch marks are generally associated with pregnancy and obesity, and can develop during rapid muscle growth from regular exercise. They first appear as reddish or purple lines, but tend to gradually fade to a lighter color. The affected areas appear empty and soft to the touch.

Stretch marks occur in the dermis, the resilient middle layer that helps the skin retain its shape. No stretch marks will form as long as there is support within the dermis. Stretching plays more of a role in where the marks occur and in what direction they run. Over time, they become whiter in color and take on a scar-like appearance. Stretch marks clearly have a different texture than normal skin.

Stretch marks can appear anywhere on the body, but are most likely to appear in places where larger amounts of fat are stored. Most common places are the abdomen, breasts, upper arms, underarms, thighs, hips and buttocks. They pose no health risk in themselves and do not compromise the body's ability to function normally.