British researchers have found that the number there can predict your total mole count. Specifically, they say, more than 11 moles on your right arm means you may have 100 moles on your entire body. And experts have long agreed that the higher your total mole count, the higher your risk of the deadly skin cancer.
"General practitioners and physicians in general have a new tool to very easily and quickly predict the total number of moles," says study researcher Simone Ribero, MD, PhD, clinical and research fellow at the department of twin research and genetic epidemiology at King's College London.
In the U.S., nearly 74,000 new cases of melanoma are expected this year, with more than 9,000 deaths.
WebMD asked Ribero and another expert to elaborate on the study and other information on moles and melanoma.
What is the difference between a mole and a freckle?
For purposes of the study, researchers defined a mole as pigmented raised tissue more than 2 millimeters (0.07 inches) in diameter, Ribero says. A mole is made up of melanocytes -- cells that produce melanin. Melanin is a pigment that gives skin its color. ''While moles are a collection of cells, freckles are a collection of pigments," says Brian Gastman, MD, a plastic surgeon specializing in skin cancer and co-director of the Cleveland Clinic Melanoma Center. "Moles, although they can come on anytime, can actually be seen at birth. Freckles usually develop later in life.''
Freckles are often flat, but a mole often can be felt when you pass your finger over it, Gastman says.
The study zeroed in on the right arm. Why?
Ribero and his colleagues analyzed data from more than 3,500 white twins, all women. Melanoma is 20 times more common in whites than in African-Americans, the American Cancer Society says.
The right arm was the best predictor. Women with more than seven moles on their right arm had a higher risk of having more than 50 total. Those with more than 11 on their right arm were more likely to have 100 total moles, meaning a higher risk of melanoma.
Ribero's team then duplicated those findings in a group of 415 healthy men and women.
The right arm finding is probably more about most people being right-handed and using that arm and hand for many tasks, resulting in more sun exposure, Gastman says.
Other things play a role in melanoma risk, too, including family history.
What should people who have a large number of moles do?
Melanoma develops from existing moles only 20% to 40% of the time.
But if you have over 50 moles on your body, you should be monitored closely by your doctor, Gastman says. And you should keep an eye on your skin yourself, performing monthly skin exams. "One of the cheapest things to do is buy a full-length mirror and have a spouse or friend keep an eye on your moles, asking if anything changed," he says.
This advice includes people who have already had melanoma. "Studies show that two-thirds to three-fourths of recurrent melanomas are found by patients or the physician by physical exam alone," Gastman says.