A birthmark is a blemish on the skin forme before birth. They are part of the group of skin lesions known as "naevi". They are not hereditary. They are soft raised swellings on the skin, often with a bright red surface, and some may look a bit like a strawberry. They are also known as "strawberry naevi" or as "infantile haemangiomas". They appear after birth, usually in the first month, and can occur anywhere on the skin. They are more of a problem when they affect the face. The cause of birthmarks is not fully understood. They are a benign overgrowth of blood vessels in the skin, and are made up of cells that usually form the inner lining of blood vessels. They are thought to occur as a result of a localized imbalance in factors controlling the development of blood vessels. Strawberry marks affect as many as one in ten newborn Caucasian babies but only about 1% of Asian and black children have them. They are particularly common in premature babies. Strawberry marks are not a sign of ill health, or associated with cancer.