The outer layer of your skin, called the epidermis, is supported by the elastic middle layer, called the dermis. The dermis is designed to stretch and bounce back, but rapid weight gain, pregnancy, and other factors sometimes cause it to tear instead. The tearing causes lines like scars called stretch marks to form on the epidermis. The marks appear as parallel reddish or purplish lines, which gradually fade. They typically occur in areas where large amounts of fat are stored.