What causes the splotches of skin pigmentation that are commonly known as birthmarks? Why do they persist on one area of the skin?

Birthmarks fall into two major categories: vascular lesions, which are spongy and consist of blood vessels, and non-vascular pigmented lesions.

Vascular lesions are caused by abnormal development of blood vessels in the skin. Although they are usually present at birth, they may also develop later in life. They may also appear suddenly, persist for a while, and then disappear again. This can happen when the body is going through a big physiological change, such as puberty or pregnancy.

As with pigmented birthmarks, there are various different ways in which vascular malformations are presented. The two most common forms are port wine stains and venous plexi  intricate networks of veins. Port wine stains are pinkish-red and flat, and may darken in colour to a purple colour several years after birth. They are usually found on the face, but may occur anywhere on the body, and they persist for life, although they can be treated by surgery or laser therapy. They may even grow larger and more conspicuous later in life, darkening, thickening or forming vascular bumps. Those that occur on the face may be associated with eye or brain problems. Venous plexi are thin and light blue, and may be flat or raised.

Haemangiomas are a third type of vascular lesion. They may develop after birth and consist of many tiny blood vessels bunched together. Haemangiomas occur in up to 2 per cent of newborns, but as many as 12 per cent of babies develop them by age one. Interestingly, they are more common in girls. Haemangiomas may change in size, and most disappear completely by age 10.