A stretch mark is actually a tear in the dermis of the skin. It occurs when skin is stretched beyond its natural ability to adjust to changes in volume accommodation. The tears are usually linear and can be very short, or as in the case of pregnancy, span many inches.
Stretch marks often affect women in pregnancy, but they can also be brought about by adolescent growth spurts, hormones, weight gain, steroid use and isometric exercises. No matter the reason, stretch marks are unwelcome.