Stretch Marks Causes: Why Skin Tears From Inside

Stretch marks are scars. StatPearls defines striae distensae as linear dermal scars that form when skin is stretched fast enough to disrupt the connective tissue underneath — collagen and elastin rupture, and the body repairs the gap with scar tissue that never matches the surrounding skin. The visible band is the repair, which is why nothing puts it back.

What is actually tearing

The break happens in the dermis, below the surface you can see. Pull it faster than fibroblasts can lay down and reorganize fiber, and the fiber network fails along lines of tension. Under the microscope the two phases look different: early marks show dilated vessels and inflammatory infiltrate around them, while mature ones show densely packed, thin, scar-like horizontal collagen bundles with a thinned epidermis over the top. That structural difference is why an early mark and an old one respond so differently to anything you put on them — see red vs white stretch marks.

Stretch alone is not the whole story

Mechanical tension explains where the marks appear; it does not explain who gets them. Hormones do part of the work, which is why the classic settings are hormonal ones — pregnancy, puberty, Cushing syndrome, corticosteroid therapy. Corticosteroids in particular reduce fibroblast activity and collagen synthesis, so the same amount of stretch does more damage. Genetics matter too: StatPearls lists family history among the risk factors, alongside connective-tissue disorders such as Marfan syndrome and the Ehlers-Danlos syndromes, where the fiber network is abnormal to begin with.

The situations that stretch skin fastest

• Pregnancy — reported in 43% to 88% of pregnancies, the single most common setting; detail in stretch marks during pregnancy
• Puberty — reported anywhere from 6% to 86% of adolescents, driven by the height spurt more than by weight; detail in stretch marks in teens
• Rapid weight gain or loss — striae are found in around 43% of people with obesity
• Rapid muscle gain from heavy training
• Corticosteroids, whether taken by mouth or applied as a strong topical for too long
• Cushing syndrome, where excess cortisol produces characteristically wide, purple striae
• Breast implants and tissue expanders, which stretch skin on a deliberate schedule

Where they show up

The distribution follows tension, not vanity: abdomen, buttocks, thighs, breasts, back, axillae and groin. StatPearls also notes striae counts were higher in Black women than in White women with otherwise comparable characteristics — one of several signals that susceptibility varies by more than behavior. Which sites dominate depends on the cause: pregnancy loads the abdomen and breasts, the adolescent growth spurt loads the lower back and thighs, and heavy lifting loads shoulders and upper arms.

Risk factors you can influence, and ones you cannot

Family history, age, sex and connective-tissue makeup are fixed. What is not fixed is the rate of change: how quickly weight is gained or lost, how aggressively training volume is added, how long a potent topical steroid stays in use. That is the whole basis of prevention advice, and it is also why the advice is so much weaker than the marketing around it — see how to prevent stretch marks.

When they deserve a doctor’s attention

Wide, purple, spreading striae in someone who is not pregnant, not growing and not lifting — and especially alongside weight gain around the trunk, easy bruising, muscle weakness or high blood pressure — should be evaluated for Cushing syndrome or steroid exposure. Striae in a young child are unusual and worth investigating. Everything else is a cosmetic decision, not a medical one.

This content is informational and does not replace medical consultation, diagnosis, or treatment.