Naevus is the medical term for a birthmark. There are two main types of naevus - pigmented birthmarks and vascular birthmarks.
A pigmented birthmark is caused when pigment cells cluster together. This usually forms a birthmark that is brown in colour and is commonly known as a mole.
Vascular birthmarks are usually red, purple or pink in colour and are caused by a collection of abnormal blood cells in or under the skin. They normally appear just after birth.
The most common form of vascular birthmark is a 'strawberry' (medically known as a heamangioma). It is a red, raised mark on the skin and usually appears on the head or neck.
Other examples of naevus include port wine stains, which are flat, purple marks on the skin affecting around one in 3000 babies; and stork marks (also known as salmon patches) a flat, pink mark that usually occurs on the forehead or neck. Stork marks are extremely common.
Around one in three babies will be born with, or will develop, a naevus. They are usually completely benign, although parents will be advised to keep an eye on the birthmark.
With the exception of port wine stains, 90 per cent of birthmarks will disappear by the time the child reaches school age. However, in a small number of cases, specialist treatment may be required.
Port wine stains are permanent marks, although some laser treatments are now available to help fade the mark.