The formation of stretch marks due to the continuous growth of the fetus during pregnancy, the abdomen continues to expand, so that the skin of the abdomen excessive stretching, subcutaneous collagen fibers eventually '' pull off ', so that the skin is formed of a special type of scar. In late pregnancy in the skin elastic fibers and collagen fibers due to traction and are subject to different degrees of damage or breakage, thinning of the skin makes depression, extrusion capillaries, in the waist and abdomen, thighs, buttocks, chest, back and other skin some pink wavy crack or purple color, often accompanied by a slight itching sensation or involved; after childbirth, the crack gradually fade into a silvery scar-like stripes. Depth or distribution lines, and also because of personal genetic constitution, the degree of weight gain varies. Stretch marks have appeared in turn is purple, or grape-colored lines.