Stretch marks form when your skin stretches faster than it can adapt, causing the supportive fibers in the deeper layer of skin to tear. The result is a visible scar that starts out red or purple and usually fades to a pale, silvery color over time. But the stretching alone isn’t the whole story. Hormones, genetics, and certain medications all play a role in whether your skin holds up or breaks down under pressure.
What Happens Inside Your Skin
Your skin has two main layers: the outer epidermis and the thicker dermis underneath. The dermis contains collagen and elastin, two proteins that give skin its strength and ability to bounce back. When skin expands rapidly, the mechanical stress exceeds what those fibers can handle. Collagen fibers break apart and then reform in thick, densely packed parallel rows, almost like scar tissue. Elastin fibers, which provide the snap-back quality, become reduced and disorganized.
At the same time, immune cells in the area release enzymes that break down elastic fibers, accelerating the damage. The genes responsible for producing collagen and a structural protein called fibronectin also become less active in the affected area. A reduction in fibrillin, a protein that acts as scaffolding for elastic fibers, has been identified as another contributing factor. The end result is a patch of skin that looks and feels different from the tissue around it: thinner, slightly indented, and often a different color.
Why Hormones Matter More Than You’d Think
Mechanical stretching is the trigger, but hormones determine how vulnerable your skin is in the first place. Cortisol, the body’s primary stress hormone, is the biggest player. It weakens the dermis by suppressing fibroblasts, the cells responsible for building collagen and elastin. When cortisol levels are elevated, your skin produces less collagen, holds fewer structural proteins, and loses some of its protective lipid barrier. In short, it becomes thinner and less resilient, exactly the conditions that make tearing more likely.
Research on stretch mark tissue shows it has significantly more hormone receptors than normal skin: roughly 2.2 times as many estrogen receptors, 1.8 times as many androgen receptors, and 1.7 times as many cortisol receptors. This means the affected areas are unusually sensitive to hormonal signals, which may explain why some patches of skin develop marks while nearby skin does not.
Pregnancy Is the Most Common Trigger
Between 55% and 90% of pregnant women develop stretch marks, making pregnancy the single most common cause. The combination of factors is almost uniquely suited to produce them: rapid abdominal expansion, a surge in cortisol and estrogen, and rising levels of relaxin, a hormone that loosens connective tissue to prepare the body for delivery. All of these hit at the same time, particularly during the third trimester when the belly grows fastest.
The marks typically appear on the abdomen, breasts, hips, and thighs. Women who are younger at their first pregnancy, who gain more weight, and who carry larger babies tend to be at higher risk. But the strongest single predictor is family history. If your mother or grandmother had stretch marks during pregnancy, your chances go up substantially.
Puberty and Growth Spurts
Stretch marks during adolescence are extremely common and often appear in otherwise healthy, non-overweight teenagers. The adolescent growth spurt combines rapid changes in height and body composition with a flood of hormones, creating conditions similar to pregnancy in terms of skin stress. In teenage boys, the marks most often appear on the buttocks, lower back, and knees. In teenage girls, the buttocks, thighs, and calves are the most common sites, along with the breasts.
These marks can catch teenagers and parents off guard because they aren’t associated with any weight problem or medical condition. They’re simply the result of the body growing faster than the skin can keep up.
Rapid Muscle Gain and Weightlifting
Bodybuilders and people who gain muscle quickly can develop stretch marks in areas most people wouldn’t expect: the shoulders, upper arms, chest, and armpits. The mechanism is the same as with any other cause. Rapid increases in muscle volume create chronic mechanical shearing and stretching that ruptures collagen fibers in the overlying skin. Combining intense weightlifting with an adolescent growth spurt, when hormones are already elevated, makes the risk even higher.
Case reports describe young weightlifters developing deep, purple marks on the deltoid area and upper arms after periods of rapid muscle growth. The marks tend to run vertically along the direction of greatest tension.
Weight Gain and Obesity
Gaining weight quickly, whether from overeating, medication side effects, or a medical condition, stretches the skin in the same way pregnancy or muscle gain does. The lower abdomen, hips, thighs, and upper arms are the most commonly affected sites. The faster the weight is gained, the less time the dermis has to adapt and produce new supportive fibers, and the more likely marks are to form.
Losing weight does not cause new stretch marks, but it can make existing ones more visible as the skin deflates and the scarred areas become more prominent.
Corticosteroid Medications
Both topical and oral corticosteroids can cause stretch marks by mimicking the effect of elevated cortisol. These medications suppress fibroblast activity and reduce collagen production, thinning the skin and making it more fragile. Potent topical steroids applied under occlusion (covered with a bandage or tight clothing) are particularly likely to cause marks in adults, even in areas that aren’t being stretched.
The risk increases with stronger formulations, longer use, and application to thinner skin such as the inner arms, groin, or armpits. Stretch marks caused by steroid overuse can sometimes be the first clue that a medication is being applied too frequently or at too high a potency.
Cushing’s Syndrome and Hormonal Disorders
When the body produces too much cortisol on its own, as in Cushing’s syndrome, stretch marks can appear even without rapid weight changes. These marks look distinctly different from ordinary ones: they tend to be wide (often more than a centimeter across) and a deep purple or reddish-violet color. By comparison, stretch marks from normal weight gain or pregnancy are typically narrow and pink or pale. If you notice unusually wide, dark purple marks appearing without an obvious cause like pregnancy or rapid weight change, it’s worth having your cortisol levels checked.
Genetics Set the Baseline
Your genes play a major role in whether you develop stretch marks and how severe they are. Large-scale genetic studies have identified several specific gene variants linked to stretch mark risk. A variant near the elastin gene (ELN) is one of the strongest associations found so far, which makes sense given elastin’s central role in skin flexibility. Other implicated genes affect fibronectin production, the extracellular matrix that supports skin structure, and even obesity-related pathways that influence how and where the body stores fat.
A positive family history, particularly a mother or grandmother with stretch marks, is one of the most consistent risk factors across studies. This means two people can go through identical experiences (same amount of weight gain, same pregnancy, same growth spurt) and end up with very different outcomes based largely on their inherited skin characteristics. Some people simply have skin that is more resilient to rapid stretching, while others are genetically predisposed to dermal tearing under relatively modest stress.
Why Some Marks Are Red and Others Are White
Fresh stretch marks, called striae rubrae, appear red, pink, or purple because of inflammation and increased blood flow in the damaged area. Over months to years, the inflammation subsides and blood vessels recede. The marks transition to striae albae: flat, pale, slightly shiny scars that sit below the level of surrounding skin. This color shift reflects the healing process. Red marks still have active inflammation and remodeling happening beneath the surface, which is why treatments aimed at stimulating collagen production tend to be more effective during this earlier phase.
The final appearance depends on your skin tone. On lighter skin, mature marks typically look white or silvery. On darker skin, they can appear lighter or darker than the surrounding area and may be more noticeable. The texture, a slightly wrinkled or indented feel, remains the same regardless of skin color.