*What causes it?:*
Stretch marks are caused when the skin stretches beyond its breaking point resulting in thin pink or red lines to manifest. Slowly the elastic collagen layers of the middle layer of the skin where stretch marks occur, called the dermis, will begin to breakdown. This damage causes the dermal tissue to become inflamed and may cause micro bleeding and the formation of scar tissue typical of stretch marks. Consequently, rough, irregular reddish lines begin to form.
As the middle layer of skin stretches the epidermis, or exterior layer of skin, it is also forced to stretch and becomes transparent. Eventually the stretch marks from the middle layer of the skin begin to protrude through the outer layer so that the stretch marks are not covered at all by the epidermis. As the inflammation and bleeding heals, scar tissue is formed, leaving a stretch mark.
In many cases the red stretch marks will gradually fade to a silvery shade as the scar tissue loses its blood supply. Because pigment-creating cells are found in the epidermis, and because the normal epidermis has split to reveal the underlying stretch mark scar which has no pigment, the resulting silvery stretch marks are lighter than the natural skin.