Stretch marks can occur from a variety of conditions, including pregnancy and muscle growth. “When your skin expands at a rapid rate, damaging changes occur in collagen, which makes up the bulk of your skin, and elastic fibers, which allow the skin to stretch and then return to its original shape,” explains Frank Wang, M.D., an assistant professor of dermatology at the University of Michigan.

Stretch marks are like scars—the body can’t adequately repair the tissue, so the marks become permanent. There aren’t a lot of proven options for preventing or reducing stretch marks. Lasers are expensive and won’t typically fully erase the marks. Some research has shown that tretinoin, a cream used to treat acne, can help reduce stretch marks when they first occur, but you need to use it for six months (and not during pregnancy), and it can be irritating, he adds.