How are birthmarks formed? While the exact process changes according to the type of birthmark, most are formed by a cluster of blood vessels or pigment in the skin. The marks cannot be caused or prevented by any specific behavior by a pregnant mother.

What Are Birthmarks?
Birthmarks can form before or a few weeks after birth. The can be any size and are found in black, brown, red, purple or blue hues. Flat or raised, rounded or jagged, each birthmark is unique to the individual. Doctors do not know what causes most birthmarks, unless they happen to be a symptom of a bigger problem. The two main types of birthmarks are vascular and pigmented.

What Are Vascular Birthmarks?
Vascular birthmarks tend to have a red, blue or purple hue. They are caused when blood vessels form incorrectly; these birthmarks are usually highly treatable. The three main types of vascular birthmarks are port-wine, salmon patch and hemangiomas.

• Port-wine birthmarks are dark red or purple splotches that can form anywhere on the body. The can grow as the child grows and darken over time. They do not disappear on their own and may require treatment. A port-wine birthmark needs to be checked by a doctor, as it may be a symptom of Klippel-Trenaunay or Beckwith-Wiedemann syndrome.
• Salmon patch birthmarks are subtle and small red marks that usually form on the head or neck. They may vanish with time, or remain for the child’s entire life. They usually do not grow or change color and require no treatment.
• Hemangiomas birthmarks can look red when they form at the surface of the skin, or bluish when they form below the surface of the skin. The marks can grow slightly in the first few years of life, but usually begin to fade as the child grows older. The bluish marks that form below the skin’s surface may leave a scar as they recede. Treatment is usually not needed, but available if the mark forms in a vital area, such as next to the eye.

What Are Pigmented Birthmarks?
Pigmented birthmarks can be any color, but are most often black, brown or red. These marks form from a clustering of skin pigment. They usually require no treatment, but many options are available if problems occur. The most common types of pigmented birthmarks are moles, café-au-lait spots and Mongolian spots. 

• Moles are an extremely common type of birthmark. The raised lumps can be any size, shape or color. Though most moles cause no problems, they can be removed if they are inconveniently placed, such as next to the eye, or highly susceptible to cancer.
• Café-au-lait spots are a very common type of birthmark. They are usually flat and light brown. They can be located anywhere on the body and require no treatment. However, a child with several café-au-lait spots needs to be checked by a doctor for the genetic disease neurofibromatosis.
• Mongolian spots are an uncommon type of birthmark that usually develops on people with heavily pigmented, and thus dark, skin. The bluish splotches are often located on the lower back or buttocks. They require no treatment and tend to fade quickly as the child ages.