What they look like: Flat patches that are tan or brown.
How common: Ten to 20 percent of newborns have one.
Should you treat or remove? No, they're generally too innocuous, even though they can grow as a child gets older. Having six or more cafe-au-lait spots larger than a quarter may be a sign of neurofibromatosis (a genetic disorder that causes abnormal growth of certain tissues).
Congenital nevi (moles)
What they look like: They may be brown or black, flat or raised, smooth or hairy.
How common: Most kids develop moles as they age, but only 1 percent of children have them at birth.
Should you treat or remove? Possibly. Dermatologists usually recommend removing large ones during the first years of life because there is a 5 to 10 percent risk that they will become cancerous.
What they look like: Blue or gray flat patches, often found on the lower back or buttocks.
How common: Most Asian babies and many African-American and Hispanic babies have them.
Should you treat or remove? No. They usually fade with time.
What they look like: Faint red patches on the forehead, eyelids, nose, or upper lip ("angel's kisses"), or on the back of the neck ("stork bites").
How common: Thirty to 50 percent of babies have one.
Should you treat or remove? No. These usually disappear by age 2, although ones on the neck may last longer.
What they look like: Flat pink, red, or purple spots. They grow, and often darken, with age. Ones on the forehead, eyelids, or both sides of the face can be linked to glaucoma and seizures, and they must be evaluated.
How common: Three out of 1,000 people have one.
Should you remove? Possibly. Laser therapy lightens marks on the face.