Stretch marks form when your skin stretches faster than it can adapt, causing the supportive fibers in the deeper layer of skin to tear. Between 40% and 90% of people develop them at some point, making them one of the most common skin changes humans experience. They’re not a sign of damage you did wrong. They’re a predictable response to rapid changes in body size, hormones, or both.
What Actually Happens Inside Your Skin
Your skin has two key structural proteins that keep it firm and flexible: collagen and elastin. Collagen provides strength, while elastin lets skin bounce back after being stretched. Both are produced by cells called fibroblasts, which live in the dermis, the thick middle layer of your skin.
When your body grows or expands quickly, the dermis gets pulled beyond its capacity. The collagen and elastin fibers tear rather than stretch, leaving behind a visible scar-like line on the surface. This isn’t a surface wound. It’s structural damage happening underneath, which is why stretch marks have a different texture than the surrounding skin and why they’re notoriously difficult to reverse once fully formed.
Hormones play a major role in this process. Cortisol, your body’s primary stress hormone (and the basis for corticosteroid medications), directly suppresses fibroblast activity. That means less collagen and elastin get produced, and the existing fibers break down faster. High cortisol levels essentially make your skin thinner, less elastic, and more vulnerable to tearing, even under moderate stretching.
Pregnancy Is the Most Common Cause
Stretch marks affect roughly 55% to 90% of pregnant women. The combination of rapid abdominal expansion, hormonal shifts, and increased cortisol creates near-perfect conditions for dermal tearing. Many women assume marks won’t appear until the third trimester when the belly is largest, but research shows a significant number develop them before 24 weeks, well into the second trimester.
The abdomen is the most obvious site, but pregnancy-related marks also appear on the breasts, hips, and thighs as those areas expand and shift. The severity varies widely from person to person, largely based on genetics and how quickly the body changes.
Puberty and Teen Growth Spurts
Teenagers frequently develop stretch marks during periods of rapid growth, and many are caught off guard by them. When bones lengthen quickly or body composition shifts during puberty, the skin over certain areas simply can’t keep up. Common locations include the abdomen, chest, hips, lower back, and thighs.
These marks are completely normal and don’t indicate excess weight gain or anything unhealthy. They’re a side effect of growing fast, which is exactly what adolescent bodies are supposed to do.
Rapid Muscle Gain
Bodybuilders and people who weight train intensely can develop stretch marks when muscle size increases faster than the overlying skin can accommodate. The biceps and upper arms are especially common sites in men, since those muscles can grow substantially with targeted training. Shoulders and chest are also frequent locations.
The mechanism is the same as any other stretch mark: the skin gets pulled beyond its elastic limit, and the collagen and elastin fibers in the dermis rupture. Pacing your training and building muscle gradually gives your skin more time to adapt, which may reduce the likelihood of marks forming. But if you’re genetically predisposed, even moderate gains can trigger them.
Weight Gain and Loss
Rapid weight gain stretches the skin mechanically, and the marks tend to follow predictable patterns based on where fat deposits accumulate. The breasts, abdomen, thighs, and buttocks are the most common sites. The faster the weight is gained, the less time the dermis has to remodel and reinforce itself, making tears more likely.
Interestingly, weight loss doesn’t cause stretch marks directly, but it can make existing ones more visible. Skin that was stretched tight over a larger body may become looser, and marks that were once flat against taut skin can become more prominent as the underlying volume decreases.
Corticosteroid Use
Long-term use of corticosteroid medications, whether applied to the skin as creams or taken orally, is one of the more overlooked causes of stretch marks. These medications suppress the cells that build and maintain collagen, while simultaneously accelerating the breakdown of elastin fibers. The result is skin that becomes progressively thinner, more fragile, and far more susceptible to tearing.
The process starts surprisingly fast. Degenerative changes in the skin can begin within 3 to 14 days of starting treatment. Short-term thinning is usually reversible once the medication stops, but prolonged use can lead to permanent structural damage, including stretch marks that won’t fade. Potent topical steroids applied to the same area repeatedly are a particularly common culprit, especially on thinner skin like the inner arms or face.
Genetics Set the Baseline
Your DNA largely determines how much collagen your skin contains, how thick that collagen mesh is, and how elastic your skin is at rest. If stretch marks run in your family, you’re statistically more likely to develop them under the same circumstances. Baseline skin hydration and elasticity are inherited traits, which explains why two people can go through nearly identical pregnancies or weight changes and end up with very different skin.
People with naturally thinner collagen networks or lower skin elasticity are more prone to visible marks. This isn’t something you can meaningfully change through skincare alone, though keeping skin well-hydrated does support its flexibility to a degree.
Red Marks vs. White Marks
Stretch marks go through two distinct stages. When they first appear, they’re typically red, pink, or purple. This early stage reflects active inflammation and increased blood flow to the damaged area. Over months to years, they fade to white or silver as the inflammation resolves and the scar tissue matures.
This distinction matters because the two stages respond very differently to treatment. Early red marks are far more responsive to topical treatments like retinoids and to clinical interventions. Once marks turn white, they become much harder to improve. Most available treatments for white marks focus on repigmenting the skin or stimulating limited collagen remodeling rather than eliminating the mark entirely. If you want to minimize the appearance of stretch marks, addressing them while they’re still in the red or pink phase gives you the best chance of meaningful improvement.
Who Gets Them Most
Several factors raise your risk beyond what’s already been covered. Being female increases your likelihood significantly, partly because of pregnancy but also because hormonal fluctuations during puberty and menstrual cycles affect skin structure. Higher body mass index correlates with more stretch marks, as does having a larger baby during pregnancy. Younger skin, paradoxically, seems more susceptible during pregnancy than older skin, possibly because of the more rapid tissue changes in younger bodies.
Certain medical conditions that affect connective tissue or cortisol production, such as Cushing’s syndrome or Marfan syndrome, dramatically increase susceptibility. But for most people, stretch marks come down to a combination of how fast their body changed, what their hormones were doing at the time, and what kind of skin they inherited.