The skin consists of three key layers: epidermis (the outer layer), dermis (the middle layer) and subcutaneous or hypodermis (the deepest layer). Stretch marks form in the dermis when the connective tissue is “stretched” beyond the limits of its elasticity due to rapid expansion or contraction of the skin from sudden growth or weight gain.
The abrupt stretching causes the dermis to tear which allows deeper skin layers to show through, forming the stretch marks.
Usually, as the body grows, the strong connecting fibers in the dermis slowly stretch. However, with rapid growth, the fibers overstretch and break. Stretch marks are red or purple initially due to blood vessels showing through the tears in the dermis.
Stretch marks eventually fade to a silvery, white or glossy appearance due to the blood vessels contracting and the pale fat underneath the skin becoming visible instead.