Congenital naevi or birthmarks are essentially coloured skin markings that develop before or shortly after birth. Naevi are sometimes called hamartomas, which are disordered proliferations of cells within the tissue of origin, and are due to a developmental error.
Benign developmental skin lesions that develop later in life are called ‘acquired’ naevi.
Naevi may be derived from the outside layers of the skin (epithelial naevi) or from the deeper layers (dermal/subcutaneous naevi). Naevi are further classified based on the cell type involved. Melanocytic and vascular naevi are generally the most common types of birthmarks.
Epithelial naevi |
Dermal/subcutaneous naevi |
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Melanocytic naevi |
Vascular naevi |
Epidermal naevi
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Follicular naevi
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Other developmental defects
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Naevi are caused by visible clusters of cells in the skin. Vascular naevi are due to clusters of blood vessels, melanocytic naevi are due to clusters of pigmented skin cells (melanocytes), epidermal naevi to keratinocyte skin cells and so on. The exact cause of why these occur is unknown but it may relate to localised abnormalities of certain genes. There is no known way to prevent them.
New ideas are being explored relating to mosaicism (the phenomenon of two cell lines arising early in embryonal development), as it has been observed that many congenital naevi follow the lines of Blaschko.
See individual birthmark types for specific treatment options.
