Stretch marks or striae (singular stria), as they are called in dermatology, are a form of scarring on the skin. They first appear as reddish or purple lines, but gradually fade to a lighter shade. They are caused by tearing of the dermis, and over time can diminish but not disappear completely.

Stretch marks are generally the result of the skin stretching rapidly like in puberty, pregnancy, muscle building or being overweight.

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.

The glucocorticoid hormones responsible for the development of stretch marks affect the epidermis by preventing the fibroblasts from forming collagen and elastin fibers, necessary to keep rapidly growing skin taut. This creates a lack of supportive material, as the skin is stretched this leads to dermal and epidermal tearing. Hormonal changes and genetics influence the skin's capacity to withstand stretching forces, as do diet

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 (especially near the belly-button), breasts, upper arms, underarms, thighs (both inner and outer), hips, and buttocks. They pose no health risk in and of themselves, and do not compromise the body's ability to function normally and repair itself.

If the epidermis and the dermis has been penetrated, laser will not remove the stretch marks.
Skin subjected to more stretching force than it can handle will tear.