Armpit stretch marks form when the skin in and around your axilla stretches faster than its underlying structure can keep up with. The deeper layer of your skin, called the dermis, contains a network of collagen and elastin fibers that give skin its strength and flexibility. When those fibers are torn by rapid stretching or weakened by hormonal changes, thin, linear streaks appear on the surface. The armpit is particularly vulnerable because it sits at the junction of several muscles that can grow quickly during puberty, exercise, or weight gain.
How Stretch Marks Form in the Skin
Stretch marks are not surface-level scratches. The damage happens in the dermis, the thick middle layer of skin responsible for structural support. When skin expands rapidly, immune cells in the dermis release enzymes that break down elastin fibers. At the same time, the cells that produce collagen (fibroblasts) can’t keep pace with the demand, so the connective tissue tears rather than adapts. Tiny blood vessels break during this process, which is why fresh stretch marks look pink, red, or purple.
Over time, the body attempts to repair the damage by laying down new collagen, but it does so in dense, flat bundles that resemble scar tissue. The skin in the affected area thins out, loses its normal ridged texture, and fades to a pale, silvery white. These older marks are essentially permanent scars within the dermis. The progression from a reddish, slightly raised streak to a flat, white line typically takes several months to a couple of years.
Puberty and Growth Spurts
The most common reason teenagers develop armpit stretch marks is the adolescent growth spurt. During puberty, the body can add inches in height and significant muscle mass over a short period, and the skin around the shoulders, upper arms, and armpits simply can’t stretch fast enough. Hormonal shifts during puberty also play a role: rising levels of adrenal hormones can reduce the skin’s ability to maintain strong collagen connections, making the dermis more susceptible to tearing even with moderate growth.
The peak age of onset for these pubertal stretch marks is between 14 and 20 in males and 10 and 16 in females. They occur in healthy, non-obese individuals and don’t necessarily signal a weight problem. Cases where stretch marks appear exclusively in the armpits and upper arms are less frequently documented, but they do happen and are considered a normal variation of adolescent skin changes.
Rapid Muscle Growth
Weightlifting and bodybuilding are a well-known trigger, especially for armpit stretch marks. The chest muscles (pectorals), shoulder muscles (deltoids), and the muscles of the upper back all attach near the armpit. When these muscles grow quickly through intense training, they push outward against the overlying skin. Stretch marks tend to appear along the perimeter of the expanding muscle, so the outer edge of the chest and the front of the armpit are common locations.
The speed of growth matters more than the total amount of growth. Someone who adds significant upper-body mass over a few months is at higher risk than someone who builds the same amount gradually over a year or two. This is why stretch marks are especially common among lifters in their first year of serious training or those using performance-enhancing substances that accelerate muscle growth beyond what the skin can accommodate.
Weight Gain and Body Composition Changes
Fat deposits in the upper arm and chest area can also stretch the armpit skin enough to cause marks. This applies to general weight gain, but the armpit is a spot where even moderate increases in body fat create tension on the skin because of how the arm moves against the torso. The skin folds and creases in this area are already under mechanical stress during everyday movements like reaching overhead or swinging your arms, so added volume underneath amplifies the strain.
Hormonal and Medical Causes
Hormones have a direct effect on how resilient your skin is. Elevated cortisol, whether from chronic stress, medical conditions, or medications, weakens collagen and elastin fibers. Cortisol essentially impairs fibroblast function, meaning the cells responsible for building and maintaining your skin’s structural framework become less effective. This is why stretch marks can appear even without dramatic changes in body size.
Cushing syndrome, a condition in which the body produces too much cortisol, causes characteristic stretch marks that tend to be wider and darker than typical ones. These commonly appear on the thighs and abdomen but can develop in other areas including the armpits. If you notice wide, reddish-purple stretch marks appearing without an obvious explanation like weight change or exercise, it’s worth having your cortisol levels checked.
Prolonged use of topical corticosteroid creams is another cause. These creams are prescribed for eczema, psoriasis, and other inflammatory skin conditions, and the armpit is a common application site. Chronic use thins the skin over time, making it far more vulnerable to tearing. Because the armpit skin is already relatively thin compared to areas like the back or thighs, it’s especially sensitive to this effect.
Genetics and Family History
Your likelihood of developing stretch marks anywhere on your body, including your armpits, is strongly influenced by genetics. Research has found that the degree of physical stretching doesn’t always predict who gets stretch marks. One study of pregnant women found no association between how much the abdomen expanded and the number of stretch marks that developed, suggesting that inherited connective tissue characteristics matter more than mechanical stretch alone.
Family history of stretch marks, personal history of previous stretch marks, and race may all serve as better predictors of who will develop them than weight gain or growth rate. Some researchers have linked stretch mark susceptibility to broader connective tissue traits, noting that women who develop stretch marks during pregnancy also have a higher prevalence of vaginal lacerations during childbirth, pointing to a shared genetic tendency toward connective tissue that tears more easily under stress.
Can You Prevent or Treat Them?
Prevention is difficult. A Cochrane review of six clinical trials involving 800 women found no high-quality evidence that any topical preparation, including products containing vitamin E, hyaluronic acid, cocoa butter, or olive oil, prevented stretch marks from forming. The statistical difference between women who used these products and those who used a placebo was not significant. Keeping your skin well-moisturized may improve comfort and overall skin health, but it won’t reliably stop stretch marks from developing if your genetics and growth patterns favor them.
For muscle-related armpit stretch marks, the most practical strategy is to slow the rate of growth. Gradual, steady progression in your training gives the skin more time to adapt. This won’t eliminate the risk entirely, but it reduces the sudden mechanical stress that causes tearing.
Treatment options are more effective on newer, reddish stretch marks than on older, white ones. Topical retinoids have shown clinical improvement in recent stretch marks by stimulating collagen production, though results vary. Procedures like fractional laser therapy can improve skin texture: one study found that about 55% of participants achieved moderate improvement (50 to 75% better) in the thickness and quality of treated skin, with roughly 22% seeing marked improvement. These procedures work by triggering a controlled wound-healing response that encourages the skin to rebuild collagen in a more organized pattern. Older, white stretch marks are harder to treat because the scar tissue is already mature, but laser treatments can still improve their appearance to some degree.