What Are Stretch Marks? As you probably already know, rapid weight gain causes stretch marks. When this occurs, it causes the middle layer of skin to stretch too quickly, and the elastic fibers snap to reveal the underlying blood vessels. This is what causes the stretch mark type scar to appear. They typically look like lines on the surface of the skin. Usually, they’re red when they’re fresh and then fade into a white or silvery hue over time (as blood vessels heal). These m