Stretch marks similar to those on the thighs are what is clinically known as expansion lines, and some are also called atrophy lines, because they occur only when pregnancy occurs. They occur because the elastic fibers of the skin become brittle and break due to overstretching, and are related to weight loss, fat gain, and excessive skeletal muscle growth. They usually appear on the inner thighs, waist, and legs, and are mainly red, skin-colored, or pale white, with alternating watermelon-like striations. Most of them can become lighter with age, similar to the skin color, or even fade away, and some of them will not disappear for life. There is no impact on health and generally no treatment is needed.