Stretch marks form when your skin stretches faster than the middle layer, called the dermis, can keep up with. The dermis contains a network of collagen and elastin fibers that give skin its strength and bounce. When mechanical stress from rapid growth, weight gain, or pregnancy exceeds what those fibers can handle, they tear. The outer layer of skin stays intact, which is why stretch marks look like scars visible through a thin surface rather than open wounds.
What Happens Inside the Skin
Your skin has three layers. The outermost layer (epidermis) is thin and flexible enough to stretch without breaking. The middle layer (dermis) is thicker and denser, packed with collagen fibers that provide structural support and elastin fibers that let skin snap back into shape. Below that sits a fatty layer that cushions everything underneath.
When the dermis is forced to expand beyond its elastic limit, the collagen and elastin fibers rupture. Think of it like pulling a rubber band slowly versus yanking it: slow, gradual stretching gives fibers time to reorganize, but rapid or extreme stretching causes irreversible damage. Research on skin biomechanics shows that skin can deform about 60 to 75 percent before it ruptures entirely, with younger skin tolerating more stretch than older skin. Stretch marks don’t require that extreme level of deformation, but they follow the same principle. Once the stress exceeds what the fibers can absorb, permanent damage occurs.
After the initial tear, the body attempts to repair the damage the same way it patches any wound: by laying down new collagen. But this replacement collagen is disorganized. Instead of the neatly woven pattern found in healthy dermis, the new fibers fill in the gaps haphazardly, creating what amounts to an atrophic scar beneath the skin’s surface. The elastin that regenerates in these areas is similarly disordered. This is why stretch marks have a different texture from surrounding skin and never fully regain normal elasticity.
Why Hormones Play a Major Role
Physical stretching alone doesn’t fully explain stretch marks. Hormones, particularly a group called glucocorticoids (which includes cortisol, your body’s main stress hormone), weaken the dermis and make it far more vulnerable to tearing.
Glucocorticoids suppress fibroblasts, the cells responsible for producing collagen and maintaining the structural scaffolding of your skin. When glucocorticoid levels are elevated, fibroblasts slow down. Collagen production drops. Elastin fibers in the upper dermis become fragmented and thin, while deeper elastin fibers collapse into dense, tightly packed bundles that lose their flexibility. The skin also loses hyaluronic acid, a molecule that helps it retain water and stay supple. The combined effect is skin that becomes thinner, more brittle, and far less capable of handling mechanical stress.
This hormonal component explains why stretch marks cluster around life events that involve both physical expansion and hormonal shifts. Pregnancy floods the body with cortisol alongside rapid abdominal growth. Puberty triggers growth spurts while hormonal systems are in flux. Even people who use corticosteroid creams or take oral steroids for medical conditions develop stretch marks, sometimes without any significant weight change at all, because the medication directly weakens the dermis from the inside.
When and Where They Typically Appear
Pregnancy is the most common trigger. Estimates of how many pregnant women develop stretch marks range from about 39 to 90 percent, with the wide range reflecting differences in skin type, genetics, age, and how much weight is gained. They most often appear on the abdomen, breasts, and hips during the third trimester, when both belly size and cortisol levels peak.
During puberty, stretch marks appear in different patterns depending on sex. In adolescent males, the buttocks, lower back, and knees are the most common sites because those areas stretch rapidly during height-driven growth spurts. In adolescent females, the buttocks, thighs, calves, and breasts are more frequently affected, reflecting where the body tends to add tissue during development. These marks often show up in otherwise healthy, non-obese teenagers simply because the growth spurt outpaces the skin’s ability to adapt.
Rapid weight gain or loss at any age can produce stretch marks, as can bodybuilding that adds muscle mass quickly. The location always corresponds to where the skin experienced the most tension.
Genetics and Individual Risk
Some people stretch considerably without a single mark, while others develop them from modest changes. Genetics is a significant reason for this difference. Researchers studying the gene activity in stretch-marked skin versus normal skin have found altered expression of genes related to collagen production, elastin structure, and the overall maintenance of the skin’s structural framework. If your parents developed stretch marks easily, you’re more likely to as well, because you may have inherited skin that produces less resilient connective tissue or responds more dramatically to hormonal changes.
Skin tone and age also matter. Younger skin is more elastic overall, but the rapid hormonal shifts of youth can offset that advantage. Lighter skin tends to develop pink or red stretch marks that are more visually prominent early on, while darker skin often produces marks that appear lighter than the surrounding tone.
The Two Stages: Red to White
Fresh stretch marks, sometimes called striae rubrae, appear red, purple, or dark pink. This color comes from blood vessels in the dermis showing through the thinned, damaged skin. At this stage, the marks may feel slightly raised or itchy as the body’s inflammatory repair process is still active.
Over months to years, the inflammation fades. Blood vessels retreat. The disorganized collagen matures into pale scar tissue, and the marks transition to their final form: flat, white or silvery lines with a slightly wrinkled texture. These mature marks are effectively scars, and like all scars, they become much harder to alter once they’ve fully settled.
Can You Prevent Them?
Prevention is difficult because stretch marks involve forces mostly outside your control: how fast you grow, your hormone levels, and your genetic skin composition. That said, some topical products show modest benefit when used consistently during pregnancy.
Creams containing centella asiatica, a plant extract that stimulates fibroblast activity and collagen production, have the strongest evidence. In one controlled trial of 80 pregnant women using a cream with this ingredient, 34 percent of the treatment group developed stretch marks compared to 56 percent in the placebo group. Women in the study who had a history of stretch marks from puberty reported even better results. Some moisturizers combining hyaluronic acid with vitamins and fatty acids have also shown reduced stretch mark rates, though researchers note that these products were applied with massage, making it unclear whether the benefit came from the cream itself or from the physical stimulation of blood flow to the skin.
Hyaluronic acid used on its own has only weak evidence for prevention. Cocoa butter and plain moisturizers, despite their popularity, have not performed better than placebo in clinical trials. Keeping weight gain gradual when possible and staying well hydrated supports skin health generally, but neither guarantees prevention if your genetics and hormones are working against you.