If you've ever stepped out of the shower and caught your reflection in the mirror to see an unfamiliar streak forming across your stomach, arms, butt or any other part of your body (and been surprised as a result), take some comfort in knowing you're not alone. Celebrities like Kourtney Kardashian, Ashley Graham and more have showed off their stretch marks.

After all, stretch marks - medically known as striae - impact many of us. "Although more common in females, these streak-like skin changes affect up to 90% of people," explains Samuel Lin, MD, an associate professor of surgery at Harvard Medical School and a double board-certified plastic surgeon who practices in Boston.

What are stretch marks? 

As the name implies, stretch marks appear as narrow streaks or lines wherever one's skin has been stretched rapidly. Such stretching can lead to the breakdown of elastic skin tissue or the "tearing of the collagen in the dermis - the middle layer of skin," says Dustin Portela, DO, a board-certified dermatologist and founder of Treasure Valley Dermatology in Boise, Idaho. He explains that stretch marks usually initially appear as reddish or purplish lines that feel slightly raised or itchy. "But over time, they usually fade to a lighter color and become flat," he says. In various stages, they may even feel depressed compared to the surrounding skin.

Though stretch marks can appear anywhere skin has stretched significantly, "they often appear on the abdomen, buttocks, thighs, breasts and hips," explains Lin. 

What causes stretch marks?

As with any abnormality, stretch marks can be caused by multiple factors. "This scarring can be caused from any rapid growth that causes the skin to stretch too quickly," says Debra Jaliman, MD, a board-certified dermatologist and assistant clinical professor of dermatology at Mt. Sinai School of Medicine in New York City. She says such causes may include "rapid weight gain or rapid muscle growth from weightlifting."

Other potential culprits include genetics, growth spurts or hormonal changes. Regarding the latter, Portela explains that certain hormones "can weaken the skin's collagen, making it more prone to stretch marks." Indeed, increased hormone levels is one of two top reasons stretch marks are so often associated with pregnancy, per the American Pregnancy Association. The other pregnancy-related cause is the same as any sudden weight gain: the skin not having enough time to adjust, such as what happens when "a woman's abdomen expands to accommodate a growing fetus," offers Portela. 

Lin says that stretch marks may also appear as a side effect of medications like steroids, injectable contraceptives and anti-retroviral protease inhibitors. "Patients should be aware that medications can cause rapid weight changes or may affect skin quality, possibly resulting in striae afterwards," he says.

Certain medical conditions such as Cushing’s syndrome, chronic liver disease, and some cases of diabetes have also been connected to stretch marks. Because of these possible connections, Lin advises consulting with your primary care physician "if you notice widespread stretch marks in addition to other symptoms including sudden weight gain, excessive hairiness, excessive thirst, brittle bones or easy bruising." 

Are stretch marks dangerous? 

Aside from potentially being a symptom of a few specific medical conditions, stretch marks aren't threatening or harmful and do not cause any known medical problems. "Stretch marks are not dangerous in any way," says Jaliman.

Stretch marks can be unsightly, however, "so they are primarily a cosmetic concern for most individuals," says Portela. For such people, there are a number of remedies that may be helpful for treating stretch marks. These include the use of retinoid creams, microneedling and laser therapies; though a Mayo Clinic report notes that such treatments "can make them fade, but they may never completely disappear."  

More:Pregnant Ashley Graham's nude photo of stretch marks wins praise from fans and celebrities

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