Stretch marks form when your skin stretches faster than it can adapt, causing structural damage in the deeper layer of skin called the dermis. They affect 50% to 90% of people at some point in life, and while they’re most associated with pregnancy, they can show up during any period of rapid body change.
What Happens Inside Your Skin
Your skin gets its ability to stretch and snap back from two types of protein fibers: collagen (which provides structure) and elastin (which provides flexibility). These fibers sit in the dermis, the thick middle layer of your skin that you can’t see from the surface. When the skin is forced to stretch too quickly, immune cells in the dermis release enzymes that break down elastin fibers. Once elastin is destroyed, collagen fibers reorganize in a disorderly way, and the result is a visible “break” in the skin’s connective tissue.
Think of it like pulling taffy too fast. Slow, steady stretching gives the fibers time to rearrange. Rapid stretching tears them apart. The damage happens below the surface, which is why stretch marks feel slightly indented compared to surrounding skin and why surface-level treatments have limited effect.
Why Stretch Marks Change Color Over Time
Fresh stretch marks, sometimes called striae rubra, start out red, pink, or purple. That color comes from inflammation and increased blood flow to the damaged area. Over months to years, the marks transition into their final form: pale, slightly shiny, and flattened. At this stage they look more like thin scars, and the color fades to white or silver because blood flow to the area has decreased and the skin has thinned. Treatments tend to be more effective during the early red or purple phase, when the tissue is still actively remodeling.
Pregnancy
About 60% of first-time mothers develop stretch marks, typically on the belly, breasts, hips, and thighs. But not all pregnant women are equally at risk. A large study published in the Journal of the American Academy of Dermatology found that age is the single strongest predictor: 84% of women under 20 developed stretch marks during pregnancy, compared with just 24% of women 30 and older. Women under 20 were roughly 25 times more likely to develop them than women over 30, even after accounting for other factors.
Pre-pregnancy body weight also matters. For every one-unit increase in BMI before pregnancy, the odds of developing stretch marks rose by about 25%. Higher starting weight means more skin stretching as the belly grows, and it may also reflect differences in skin composition. Hormonal shifts during pregnancy further soften connective tissue to prepare the body for delivery, which makes the dermis more vulnerable to tearing.
Puberty and Growth Spurts
Teenagers develop stretch marks more often than most people realize. During puberty, bones and muscles can grow faster than the overlying skin can keep up with, especially during peak growth spurts. In adolescent girls, the marks most commonly appear on the thighs, buttocks, and breasts. In adolescent boys, they tend to show up on the lower back and hip area, where the torso is expanding rapidly. These marks are completely unrelated to weight gain and can appear in lean, active teenagers whose bodies are simply growing fast.
Rapid Muscle Growth and Weight Changes
Bodybuilders frequently develop stretch marks on the biceps, shoulders, and chest, particularly when they add muscle mass quickly through intense training programs. The mechanism is the same as in pregnancy or growth spurts: the muscle underneath expands faster than the skin above it can accommodate, and the collagen and elastin fibers tear. People who gain a significant amount of body fat over a short period face the same problem on the belly, hips, and upper arms. Losing weight doesn’t remove existing stretch marks, though they may become less noticeable as the skin relaxes.
Hormones and Cortisol
The hormone cortisol plays a direct role in stretch mark formation. Cortisol weakens elastic fibers in the dermis, making the skin more prone to tearing under stress. This is why stretch marks are so common during periods of natural hormonal change like puberty and pregnancy, when cortisol levels fluctuate.
Cushing syndrome, a condition where the body produces too much cortisol over a prolonged period, causes distinctive wide, pink or purple stretch marks on the stomach, hips, thighs, breasts, and underarms. These marks tend to be darker and more prominent than typical stretch marks and often appear alongside other symptoms like a rounded face and a fatty deposit between the shoulders. If you develop large, deeply colored stretch marks without an obvious cause like weight change or pregnancy, that pattern can signal an underlying hormonal issue worth investigating.
Steroid Creams and Medications
Potent topical steroid creams, the kind prescribed for eczema, psoriasis, or fungal infections, can cause stretch marks in as little as three weeks of regular use. These medications thin the skin by reducing collagen production and constricting blood vessels in the dermis. The strongest formulations carry the highest risk, especially when applied to areas where skin is already thin, like the inner arms, groin, or armpits. Misuse of steroid-containing creams, particularly combination products used without medical supervision, is a recognized cause of severe stretch marks and even skin ulceration.
Genetics and Skin Type
Some people stretch their skin dramatically and never develop a single mark. Others get them from modest changes. The difference is largely genetic. Researchers have identified variations in several genes that influence stretch mark risk, including genes responsible for producing elastin, collagen, and fibronectin (a protein that helps organize connective tissue). If your parents have prominent stretch marks, you’re more likely to develop them yourself, regardless of how carefully you manage weight or skin care.
Skin color doesn’t determine whether you’ll get stretch marks, but it does affect how visible they are. On darker skin tones, stretch marks often appear lighter than surrounding skin once they mature, making them more noticeable. On lighter skin, mature marks blend in more easily but may be more visible during the early red or purple phase.
Where Stretch Marks Typically Appear
- Belly and hips: most common during pregnancy and weight gain
- Breasts: common during puberty, pregnancy, and rapid weight change
- Thighs and buttocks: frequent in adolescent girls and during weight fluctuations
- Lower back: the most common location in adolescent boys during growth spurts
- Upper arms and shoulders: typical in bodybuilders and people who gain muscle rapidly
The location depends almost entirely on where the stretching is happening. Stretch marks follow the lines of tension in the skin, which is why they usually run perpendicular to the direction of growth.