The open-access journal PLoS Medicine reported on Tuesday that two prospective cohort studies–large population-based studies that follow subjects over time and monitor for disease development–found that the risk of developing breast cancer increased slightly with increases in number of moles on the skin of women.

In one of the studies, researchers at Harvard and Indiana Universities followed over 74,500 female nurses from 1986 through 2010 in a large, nation-wide cohort study called The Nurses’ Health Study. The ages of the women at the beginning of the study ranged from 40 to 65 years. During the 24-year study period, nearly 5,500 women were diagnosed with invasive breast cancer. Women with 15 or more moles on their left arm were 35 percent more likely to develop breast cancer than those who reported no moles at the beginning of the study. Those with fewer than 15 moles exhibited a slightly higher risk than those without moles.

Among women with no moles, 8.5 percent developed breast cancer. Among those with one to 14 moles, 11.4 percent developed breast cancer and among those with 15 or more moles, 11.4 percent developed the disease.

A group of French researchers reported in the same issue that women in France, ages 39 to 66 years at the time of enrollment in the E3N Teachers’ Study Cohort in 1989–1991, exhibited similar links between moles and breast cancer risk. The researchers followed nearly 90,000 women through 2008 and by that time, a total of almost 6,000 had developed breast cancer. Women with the most moles had a 13 percent higher risk of developing breast cancer. However, the association, found only in premenopausal women, was weak and could not hold after controlling for non-cancerous breast problems and family history of breast cancer–both factors that can increase the risk of breast cancer.

Barbara Fuhrman, assistant professor at the University of Arkansas for Medical Sciences in Little Rock and co-author of a commentary that accompanied the two research reports in the same issue of the journal, said that women with skin moles should not panic because the results probably do not reflect accurately on the risk of individual women.

“[The moles] could be a marker of lifetime exposure to estrogen,” said Fuhrman.

The U.S. researchers found in a subset of their subjects that estrogen and testosterone levels were higher in the women with 15 or more moles. They speculated that the hormone levels may bridge the gap between moles and elevated cancer risk. Even so, Fuhrman stressed that the increase in risk found by the study was overall very small.