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, the Medical Daily reported.

One of the two studies, conducted by American scientists, found that women with more than 15 moles on their arm (nevi) 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.

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 Medical Daily with reference to Health Day.

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.