Pigmented nevi (moles) are growths on the skin that usually are flesh-colored, brown or black. Moles can appear anywhere on the skin, alone or in groups. Moles occur when cells in the skin grow in a cluster instead of being spread throughout the skin. Moles may darken after exposure to the sun, during the teen years and during pregnancy.

In most cases, health care professionals can diagnose birthmarks based on the appearance of the skin. If a mole exhibits potentially cancerous changes, a biopsy may be performed.