Stretch marks are the result of the rapid stretching of the skin due to sudden growth or weight gain. This stretching causes the middle layer of skin (the derm) to tear, allowing the deeper skin layers to show through, forming stretch marks. The dermis contains strong, inter-connected fibers that allow your skin to stretch as your body grows. However, rapid growth can over-stretch and break the fibers. The tears in the dermis allow the blood vessels below to show through. This is why stretch marks are often red or purple when they first appear.