We have three layers of skin: the epidermis, or outer layer; the dermis, or middle layer; and the subcutaneous stratum, or deepest layer. Stretch marks (also known as stria atrophica and striae distensae) occur in the dermis, the elastic, resilient middle layer that allows skin to retain its shape. If stretch marks formed on the skin's surface, they'd be much easier to treat. When the dermis is constantly stretched over time, the skin becomes less elastic and the connective fibers break. The result is the markings we know as stretch marks.
Depending on your natural skin coloring, stretch marks begin as raised pink, reddish brown or dark brown striations that then turn a brighter violet or purple. Gradually these bright marks flatten and fade to a color a few shades lighter than your natural skin tone. They usually become less noticeable over time.
Stretch marks can appear anywhere on the body where the skin has been stretched (often as a result of weight gain). They're most likely to appear in places where fat is stored: the abdomen, breasts, upper arms, thighs and buttocks. They pose absolutely no health risk and don't compromise your body's ability to function healthily; they are purely a cosmetic issue and need to be treated accordingly.