How is a birthmark made?

A birthmark is formed during fetal development when skin cells grow abnormally or when blood vessels cluster together. They can be classified into two main types: pigmented birthmarks, which are caused by an excess of pigment cells, and vascular birthmarks, which result from abnormal blood vessel formations. The exact cause of why some birthmarks develop is not fully understood, but genetic and environmental factors may play a role. Most birthmarks are harmless and may fade or change over time.