The Art and Science of Beautiful Skin.
Birthmarks are made of vessels or pigmented cells. They are not always visible at birth. Moles that are bigger than a half inch are called congenital nevi and should be watched for change. Not all change is bad but a physician should be familiar with a congenital mole and be able to analyze a change that occurs in it.
Birthmarks made from vessels (usually reddish or bluish) seldom cause problems unless they grow rapidly in an infant. They can often be treated with a laser. Some pigmented marks are not moles and show up when a child is young. These pigmentary alterations are seldom a problem and the pediatrician will watch them for change. One common type of birthmark in this category is a cafe-au-lait macule.
For more information, visit the American Academy of Dermatology Association.