Causes of vascular birthmarks
NHS Choices Medical Reference
The cause of vascular birthmarks is not fully understood. Birthmarks are not usually inherited.
Haemangiomas are caused by an overgrowth of tiny blood vessels under the skin, although it is not understood exactly why this happens.
Port wine stains
Port wine stains are caused by a problem with the tiny blood vessels, known as capillaries, in the area where the mark forms.
Normally, capillaries can narrow (constrict) or widen (dilate). Dilated vessels cause more blood to flow to the skin to allow the blood to cool when you are hot. Constricted vessels prevent too much blood reaching the surface of the skin when you are cold.
In the case of port wine stains, the capillaries remain dilated. This results means that blood is constantly supplied to the skin in that area, which makes it permanently red or purple in colour.
It is thought that port wine stains occur because the nerves that control the widening or narrowing of the capillaries do not function properly, or there are not enough of them.
Port wine stains are sometimes related to other conditions, such as Sturge-Weber syndrome and Klippel-Trenaunay syndrome (see Complications for more information).
Salmon patches (stork marks) are caused by a collection of tiny blood vessels under the skin. They are sometimes more noticeable when a baby cries.
Blood vessels: Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
Congenital: Congenital means a condition that is present at birth. The condition could be hereditary or develops during pregnancy.
Blood: Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.