Although not fully understood, it is believed that overstretching at the skin dermis causes collagen and elastin fibers to rupture, resulting in scars (stretch marks) during the healing process. The hormone, cortisone may also have a role. An increase in cortisone levels can increase stretch marks by reducing skin pliability. It does this by inhibiting fibroblasts from producing collagen and elastin fibers, which keep rapidly growing skin taut.