What causes pigmented birthmarks?

What causes pigmented birthmarks?

The cause of pigmented birthmarks is not known. However, the amount and location of melanin (a substance that determines skin color) determines the color of pigmented birthmarks. Cafe-au-lait spots might be a normal type of birthmark, but the presence of several cafe-au-lait spots larger than a quarter might occur in neurofibromatosis (a genetic disorder that causes abnormal cell growth of nerve tissues). Moles occur when cells in the skin grow in a cluster instead of being spread throughout the skin. These cells are called melanocytes, and they make the pigment that gives skin its natural color. Moles might darken after exposure to the sun, during the teen years, and during pregnancy.