Keloids form when the body’s wound-healing process goes into overdrive, producing far more collagen than needed and creating raised, firm scar tissue that grows beyond the edges of the original injury. This overgrowth is driven by a combination of genetics, the type and location of the injury, hormonal shifts, and disrupted signaling between cells. Understanding these causes can help you recognize your own risk and avoid common triggers.
How Normal Healing Goes Wrong
When skin is injured, your body sends specialized cells called fibroblasts to the wound site. These cells produce collagen, the structural protein that knits skin back together. In normal healing, the process slows down once the wound is closed. In keloid-prone individuals, the fibroblasts never get the message to stop.
The key culprit is a growth signal called TGF-beta, which acts like an accelerator pedal for collagen production. In keloid tissue, fibroblasts are hypersensitive to this signal. They keep producing collagen and other structural proteins long after the wound has closed, and they resist the normal self-destruct signals that would clear them out. A small regulatory molecule called miR-21 is also overactive in keloids, and it disables the body’s natural brake on this process, creating a self-reinforcing loop of scar growth.
The result is a mass of dense, disorganized collagen that pushes outward from the original wound. This is the defining feature that separates a keloid from a hypertrophic scar: a hypertrophic scar stays within the boundaries of the original injury and often fades over time, while a keloid spreads beyond those borders and does not regress on its own.
Skin Injuries That Trigger Keloids
Almost any form of skin damage can set off keloid formation in someone who is susceptible. The most common triggers include:
- Surgical incisions
- Ear and body piercings
- Burns
- Acne (especially deep, cystic acne)
- Vaccinations and injections
- Insect bites, scratches, and minor bumps
In some cases, keloids appear without any identifiable skin injury at all, which suggests that even microscopic or unnoticed damage to the skin can be enough. The severity of the trigger doesn’t always predict the severity of the keloid. A simple earlobe piercing can produce a scar the size of a marble, while a larger surgical wound might heal cleanly.
Genetics and Family History
Keloid tendency runs strongly in families. If one or both of your parents developed keloids, your risk is significantly higher. Researchers have identified several chromosomal regions and gene variants linked to keloid susceptibility, though no single “keloid gene” has been pinpointed.
Genome-wide studies in Japanese and Chinese populations have found keloid-associated markers on chromosomes 1, 3, and 15. One gene on chromosome 15, called NEDD4, showed a significant association with keloid risk in both populations. Variants in the PTEN gene have also been linked to higher keloid risk in the Chinese Han population. In a large Yoruba family from West Africa, researchers identified a specific variant in the ASAH1 gene that tracked with keloid development across generations.
Several of the genes implicated in keloid susceptibility are involved in the same TGF-beta signaling pathway that drives excessive collagen production. Variants in SMAD3, SMAD6, and SMAD7, all of which regulate that pathway, have been studied as potential contributors. This means the genetic predisposition isn’t random; it maps onto the biological machinery that controls scar formation.
Who Is Most at Risk
People with darker skin tones are disproportionately affected. Keloid rates among individuals of African, Asian, and Hispanic descent range from 4.5% to 16%, compared with much lower rates in Caucasian populations. The reasons for this disparity are not fully understood but are thought to involve differences in fibroblast behavior and genetic susceptibility rather than skin pigmentation itself.
Keloids most commonly develop between puberty and age 30, which aligns with the hormonal factors discussed below. They affect both men and women, though certain trigger types (like earlobe piercings) create different patterns of presentation between sexes.
Where on the Body Keloids Form
Keloids show a strong preference for certain body sites. In a review of 1,000 keloid patients, 34% of keloids occurred on the chest (specifically the breastbone area). Other common locations were the shoulders and upper arms (17%), the rest of the upper limbs (13%), lower limbs (10%), and earlobes (9%). The upper back and posterior neck are also frequent sites.
The prevailing theory is that areas of high skin tension and mechanical stress are more vulnerable because the constant pulling on a healing wound keeps fibroblasts active longer. Your chest and shoulders move with every breath and arm movement, generating ongoing tension across any healing scar. However, earlobe keloids complicate this theory since earlobes are low-tension tissue. Some researchers believe that in those cases, pieces of skin tissue get trapped during healing and continue to grow in predisposed individuals, regardless of tension.
If you’ve had a keloid on one body site, the same site on the other side of your body carries a similar risk. This site-specific vulnerability is worth considering before piercings, tattoos, or elective procedures in high-risk areas.
Hormonal Changes and Keloid Growth
Hormonal fluctuations during puberty and pregnancy appear to accelerate keloid formation and worsen existing scars. Clinical reports document cases where a flat, stable scar transformed into a thick keloid during puberty, and then grew again dramatically during pregnancy, reaching the size of a tennis ball in one documented case. In another patient, a well-settled keloid became painful, itchy, thicker, and wider early in the first trimester of pregnancy.
These observations suggest that the hormones surging during puberty and pregnancy, particularly estrogen, may stimulate fibroblast activity or amplify the signaling pathways that drive collagen overproduction. The pattern is consistent: scars that have been stable for years suddenly reactivate during hormonal transitions, and no other risk factor explains the timing. For people with existing keloids or a known tendency, this is worth keeping in mind during pregnancy planning.
What Keloids Feel Like
Keloids are not just a cosmetic concern. Between 50% and 90% of keloid patients experience pain, itching, or both. The itching tends to be worse at the edges of the keloid rather than the center, likely because nerve fibers are more densely packed at the growing border of the scar. As the keloid expands, it pulls on surrounding nerves, creating chronic mechanical stimulation that produces persistent pain and itch.
Large keloids can also restrict movement if they form near joints or across areas that need to flex. The skin over a keloid is firm and inflexible, and the scar itself may be darker than the surrounding skin. Over time, keloids can continue to grow slowly, sometimes for years after the initial injury, which distinguishes them from hypertrophic scars that typically stabilize within 6 to 12 months.
Keloids vs. Hypertrophic Scars
The two are frequently confused, but the distinction matters for treatment and prognosis. A hypertrophic scar is raised and thickened but stays within the boundaries of the original wound. It often improves on its own over months to years. A keloid, by definition, extends beyond the original wound margins. It does not regress spontaneously and tends to recur after removal. If a raised scar has not spread past the edges of the injury, it is not a keloid, regardless of how thick or uncomfortable it is.