Moles can be cancerous or benign. Most moles appear in the first 20 years of life, though some may develop in later years. It is not uncommon for an adult to have between 10 and 40 moles, according to the U.S. National Cancer Institute. Dermatologists classify moles, called nevi, in several different categories and they result from a variety of causes.
Melanocytes
Some moles are present at birth and are the result of a concentration of melanocytes in one area. Melanocytes produce the pigment that gives skin its color, so when they are concentrated in a cluster, the result is a pink, light brown or dark brown patch. About 1 in 100 babies born have at least one congenital nevus, as they’re called, according to the American Osteopathic College of Dermatology. Increased exposure to the sun can darken congenital nevi over time.
Giant congenital nevi measure more than 4 inches and occur once in every 20,000 births. Risk of melanoma developing from giant nevi is estimated at about 6 percent, according to the AOCD.
Genetics
Like all moles, the appearance of an atypical nevus is the result of the gathering of melanocytes in a cluster. The cause of this process is not certain, though the American Academy of Dermatologists cites excessive sun exposure as a possible catalyst. Development of atypical nevi tends to follow family lines, so the likelihood of a genetic component is high.
They are characterized by irregular borders, sometimes cut with notches, and the circumference is generally flush with the skin. They possess many of the earmarks of melanoma, and patients demonstrating any of the risk factors—family history of melanoma, the presence of multiple atypical nevi or a previous bout of skin cancer—should have atypical moles examined, according to the AAD.
Sun Damage
Most moles that develop during a lifetime are harmless and small. These are acquired nevi, and they are thought to be the result of sun damage early in life, though acquired nevi on the face are thought to be genetic, according to the AOCD. Acquired nevi appearing after age 50 pose some concern, and should be looked at but both the AAD and AOCD consider most acquired nevi to be at low risk for developing into melanoma.
