Stretch marks are caused by rapid stretching of the skin due to growth or weight gain. The skin is made up of three layers- the epidermis (outermost), dermis (middle layer) and hypodermis. The epidermis creates a waterproof outer covering and is what gives you color ie contains melanin. The dermis is much thicker, contains tough connective tissue, hair follicles and sweat glands while the innermost layer is made of fat and connective tissue.
It is the dermis that allows your skin to stretch as your body grows. However, rapid growth can cause the skin to over-stretch and break the tissue fibres. The tears in the dermis allow the blood vessels below to show through, which is why stretch marks are often red or purple when they first appear.When the blood vessels eventually contract, the pale-colored fat in your hypodermis will be visible, and your stretch marks will change to a lighter color.
Most stretch marks fade over time. There are some creams and oils that are said to reduce stretch marks by making them fade faster. Laser treatment and cosmetic surgeries can also be used, but are very expensive