Stretch marks, or striae, are a type of skin scarring that affects the dermis and epidermis layers of the human skin. First appearing as red or purple lines on the skin, the scars usually fade to silver-pink or white within a few years. The striae are believed to be a result of tearing to the dermis, and are caused by several different conditions and factors.

The human skin is made up of three layers. The visible layer is called the epidermis, and is where the marks appear. The middle layer, called the dermis, is rich in collagen, which is meant to keep the skin taut. In periods of rapid growth, collagen production may not be able to keep up with the stretching of skin, and the dermis can tear and lose elasticity. This tearing can result in discolored scarring of the skin.

One of the most common groups affected by stretch marks is women who are pregnant. Studies suggest that between 70% and 90% of pregnant women develop the scarring, usually around the seventh month of pregnancy. These types of stretch marks are most often found in the areas specifically related to pregnancy growth, such as the abdomen and breasts.

In adolescence, growth spurts or sudden weight gain can cause red and purple types of stretch marks in places where fat is stored. Typical areas for growth-spurt related types of stretch marks are the back, upper arms, breasts, hips and thighs. Stretch marks can also be formed by quick muscle growth, commonly seen in young athletes. Some studies have shown that as many as 70% of adolescent girls and 40% of adolescent boys are affected by striae.

Bodybuilding or weightlifting can lead to muscle-based types of stretch marks. In these cases, marks usually appear on the growing muscles, such as the biceps or calf muscles. Users of steroids such as cortisone are particularly subject to these types of stretch marks, because of the effects of steroid drugs. Steroid use allows the rapid build of muscle in the body, which may cause skin to stretch at an increased rate, leading to striae.

The science behind different types of stretch marks is not certain. While the stretching-skin theory is commonly accepted, some experts argue that a hormone called glucocorticoids is responsible for the scarring. This hormone is secreted by the body in high concentration in adolescents, pregnant women, body builders and by those suffering from obesity, which are all groups subject to some types of stretch marks. Experts suggest that the hormone prevents collagen from forming, causing a breakdown in elasticity. While the stretch marks may appear in areas subject to sudden growth, it is possible that the scarring is caused by the hormone increase, rather than the growth itself.

Most types of stretch marks usually fade with time, but are unlikely to disappear entirely. Although various oils and creams are suggested to help prevent stretch marks from appearing, none have proved consistently effective. Laser treatments, chemical peels and ointments are all used to reduce the appearance of already formed striae, but no treatment has been proven to completely remove them.