There are cracks in the waist and buttocks. If you are a woman who has given birth to a child, the cracks in the waist and buttocks are generally stretch marks, while the cracks in the waist and buttocks of teenagers are generally growth lines formed by overgrowth. There are cracks (marks) on the waist and buttocks, including thighs. There are two situations: stretch marks or growth lines. Stretch marks are caused by the enlargement of the uterus during pregnancy and the rapid expansion of the pregnant woman's abdomen, so that the elastic fibers and collagen fibers of the skin are damaged or broken to varying degrees by external forces, and the skin becomes thinner and thinner, resulting in the rupture of collagen fibers and even elastic fibers rich in subcutaneous tissue. There will be some wavy lines with different widths and lengths on the abdominal skin, buttocks and thighs, thus forming stretch marks. The growth line usually occurs in the growth period of teenagers. During the growth period of teenagers, due to the rapid growth of bones and muscles, the rapid growth of bones and muscles exceeds the stretching speed of skin, and the elastic fibers of dermis are pulled off, thus forming a growth line.