According to WebMD and Mayo Clinic, clusters of skin cells called melanocytes cause moles. Normally, these skin cells are spread evenly. Melanocytes produce melanin, the pigment that gives skin its ordinary color. Sun exposure and pregnancy are some events that may cause moles to get darker.
Mayo Clinic notes that the majority of moles warrant no medical attention, but some odd-looking moles, called dysplastic nevi, may be more prone to cancer. WebMD explains that dysplastic nevi are bigger than a typical pencil eraser and are unevenly colored and irregularly shaped. A person who has at least 10 dysplastic nevi is 12 times more at risk at developing melanoma than someone who has fewer or no dysplastic nevi.
Congenital nevi are another type of mole that sometimes becomes cancerous. About 1 in 100 people are born with congenital nevi. By the time a person reaches adulthood, he normally has had 10 to 40 moles, according to WebMD. As he ages, some moles change. They may become more three-dimensional, grow hairs and shift colors. Other moles may gradually vanish, while some stay the same. People worried about a mole should monitor any changes in size, color and shape. Moles on areas of the skin that get frequent sun exposure need extra attention as well.