Stretch marks aren't unusual in teens (they show up in boys as well as girls). In fact, they're considered a normal part of puberty. When the skin is stretched by rapid growth, synthesis of collagen (the fibrous protein that gives skin its strength) is disturbed, leading to the appearance of these striations.
Although stretch marks can be very upsetting - especially when they appear during the self-conscious teen years - they usually fade and become much less noticeable over time. While your daughter waits for that to happen, she can try using self-tanning creams or lotions or body makeup to help cover the marks. You just apply these products when you're wearing a bathing suit or shorts (some are water resistant). Tanning itself won't help - the stretch marks are less likely to tan than surrounding skin so their appearance may actually be emphasized. Besides, tanning isn't good for the skin's long-term health.
Many products are advertised as effective methods to get rid of stretch marks, but most are expensive and ineffective. You can check them out at www.reviewspot.org. Dermatologists typically treat stretch marks with lasers, microdermabrasion (a treatment that removes the top layer of skin) and, for recent stretch marks, tretinoin cream (Retin A, Renova) which helps to rebuild collagen. This last treatment doesn't always work, can have side effects, and isn't effective for older stretch marks.
Assure your daughter that stretch marks are common among girls and boys in her age range and that hers will fade with time.
Andrew Weil, M.D.