Birthmarks are spots, blemishes or marks on the skin that are present at birth or that appear shortly thereafter. In most cases, the marks are harmless and eventually fade. Some birthmarks are present and remain nonthreatening throughout a person’s lifetime, but there are cases in which birthmarks can become harmful.

Whether a birthmark becomes a problem depends in part on the type. Here’s a brief description of the different types of birthmarks and their colors, causes and characteristics.

Café au lait spots are light coffee or brown and typically oval. Babies can be born with more than one café au lait spot and they do not fade with age. These birthmarks are caused by an increase in melanin, a substance that gives skin color.

Congenital melanocytic nevi are large black or brown moles that have an irregular shape and may be raised or flat. During puberty they can grow darker and develop hair. The risk of cancer is minimal and is related to the size of the birthmark; that is, the larger the birthmark the greater the risk. They are caused by an accumulation of melanocytes (cells that produce melanin).

Hemangiomas, also known as strawberry marks, are red and raised. Often they grow quickly during the first few months and then slow down and may fade. Hemangiomas are believed to be caused by an accumulation of cells in the lining of blood vessels. There is a slight chance hemangiomas can become infected.

Mongolian spots typically are seen in people who have dark skin. These harmless marks look like bruises and disappear by age 4 years.

Port wine stains most often appear on the face and can vary in size from a few millimeters to several centimeters. At birth the stain is pink and flat and with age the mark may darken. Port wine stains develop when the nerves that control the expanding and constriction of the capillaries are damaged and the vessels swell. Most port wine stains are harmless, but those that develop around the eyelid may need to be treated.

Telangiectatic nevi are patches of reddish skin that appear on the face or back of the neck. The birthmark is caused by the expansion of capillaries. Marks on the face usually fade while those on the neck generally remain but are hidden by hair.

Another proposed cause of birthmarks involves the placenta. It is possible that certain proteins produced by the placenta are associated with an increased risk of birthmarks.