Moles, beauty marks, and birth marks are, for most people, just another thing on their skin. And except for those few health conscious teens, many of us don’t learn about the real risk moles pose to us when it comes to skin cancer. But now, two new studies suggests that the number of moles on a person’s body may also determine their risk of developing breast cancer.

Moles are relatively common, growing on the skin of most children, and becoming lighter or darker as they age. According to the American Academy of Dermatology, almost every adult has a few moles, with 10 to 40 at various places around the body being fairly common, and normal. People who have more than 50, however, have a greater chance of developing melanoma, the most dangerous form of skin cancer.

One of the two studies, conducted by American scientists, found that women with more than 15 moles, also called nevi, on their arm are 35 percent more at risk of developing breast cancer. The other study, by French scientists, found that women who reported having “very many” moles at the beginning of the study were 13 percent more likely to develop breast cancer — a “modest association between number of nevi and overall breast cancer risk,” they wrote, according to NBC.

The American researchers speculated that the risk could come from excess estrogen, which has been linked to the both the growth and spread of breast cancer tumors, and the growth of nevi. “They could be a marker of lifetime exposure to estrogen,” said Barbara Fuhrman, an assistant professor at the University of Arkansas for Medical Sciences, according to Health Day.

However, that link may only apply to the broader population and not to an individual patient. Dr. Deanna Attai, a breast surgeon in California, told NBC that risk factors like obesity and family history are still more crucial to a person’s overall risk.  

The American study used data from the Nurses’ Health Study, which involved over 74,500 women over the course of 24 years. The researchers found that 8.48 percent of the women without nevideveloped breast cancer, whereas 11.4 percent of those with 15 or more developed it. A subgroup of women found that postmenopausal women with six or more nevi had higher levels of both estrogen and testosterone, and those without nevi. After eliminating estrogen levels as a factor, the association between nevi and breast cancer “disappeared,” a press release said.

The French study, conducted by researchers at INSERM, a biomedical and public health research institution, looked at the E3N Teachers’ Study Cohort. It involved almost 90,000 women and followed them over 18 years. Though having “very many” moles was associated with breast cancer risk, the researchers found that the association wasn’t significant after adjusting for known breast cancer risk factors, such as benign breast disease and a family history.

The researchers of both studies emphasized these limitations, and called for further research. With an estimated 232,670 new cases and 40,000 deaths this year, breast cancer is the most common type of cancer in women. Until these findings can be confirmed, the researchers said that the number of nevi on a woman’s body may only be used as a marker, or hint, to a woman’s risk — how it would improve risk estimation is another question, however.

Source: Zhang M, Zhang X, Qureshi A, Eliassen H, Hankinson S, Han J. Association between Cutaneous Nevi and Breast Cancer in the Nurses' Health Study: A Prospective Cohort Study. PLoS Medicine. 2014.