What are Stretch Marks Stretch marks are long narrow streaks, stripes silvery lines that develop on the skin due to extension or contraction after stretching of muscles. The marks initially develop as wrinkly, raised streaks that can be red, purple, pink, reddish-brown or dark brown, depending on skin color. Where and why they form? They form in epidermis layer of skin mainly in women than men in the area of stomach, buttock, breast, thigh, upper arms and lower back. Deficiency of collagen in the skin is the cause stretch mark. What are the reasons of stretch mark? Pregnancy Stress and Hormonal changes Rapid weight gain and loss Body building