Stretch marks are often the result of the rapid stretching of the skin associated with rapid growth. Stretch marks may also be influenced by hormonal changes associated with puberty, pregnancy, muscle building, hormone replacement therapy for transsexuals, etc.^[1] Medical terminology for these kinds of markings includes striae atrophicae, vergetures, stria distensae, striae cutis distensae, striae gravidarum (in cases where it is caused by pregnancy), lineae atrophicae, linea albicante, or simply striae.

They first appear as reddish or purple lines, but tend to gradually fade to a lighter range. The affected areas appear empty and are soft to the touch.^[2] 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. Stretching alone is not the cause.

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 navel), breasts, upper arms, underarms, back, 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.^[3]