The skin consists of three key layers. Stretch marks form in the dermis, or middle layer, when the connective tissue is stretched beyond the limits of its elasticity. This is normally due to rapid expansion or contraction of the skin.
As the body grows, the connecting fibers in the dermis slowly stretch to accommodate slow growth. However, rapid growth leads to sudden stretching. This causes the dermis to tear, allowing deeper layers of skin to show through.
Stretch marks eventually fade to a silvery, white, or glossy appearance, due to the pale fat beneath the skin becoming visible instead of the usual blood vessels.