Stretch marks are purplish streaks and scars that normally appear on the buttocks, hips, abdomen, breast, and arms. They are a common side effect of growing too fast. They form when the dermis (the middle elastic layer of your skin) is stretched breaking down the elasticity in your skin. As the elasticity breaks down, the skin tries to reinforce itself with collagen in the over stretched skin, thus causing stretch marks.Factors that contribute to the formation of stretch marks are:Heredity - .