Congenital Melanocytic Naevi


A congenital melanocytic naevus (CMN) is one of many different types of birthmark found in newborn babies. It is also known as a mole.

  • Congenital = present at birth.
  • Melanocytic = to do with melanocytes. Melanocytes are cells in everyone’s skin and hair that produce the pigment melanin. This is what which gives us our hair and skin colour. The cells in a CMN look most like melanocytes and they produce pigment, so they are called melanocytic.
  • Naevus = birthmark (the plural is naevi).

So CMN means brown birthmark. They can be any size, and can be single or multiple (two or more). Sometimes they are very large and very numerous.


Last updated July 2014 by Dr V Kinsler, Consultant in Paediatric Dermatology, Great Ormond Street Hospital, London, UK.

What are the symptoms?

CMNs show a number of features, including:

  • variation in size
  • colour of the CMN is often uneven and can vary over the years
  • texture: the texture of large CMNs is softer, looser and more wrinkled than normal skin
  • hairiness: CMNs are usually hairier than normal areas of skin
  • lumpiness: in large CMNs, quite often there are raised or lumpy areas
  • eczema: the skin overlying a CMN is often rather dry and itchy, and may sometimes develop eczema
  • underlying absence of fat: the presence of many melanocytes interferes with the development of the layer of fat that is normally present between the skin and underlying muscle and bone.

CMN can also be associated with other problems if they are large or multiple:

  • brain or spine problems – these can usually be detected using MRI scans, ideally in the first 6 months of life. This can be called neurocutaneous melanosis, or CMN syndrome
  • a risk of melanoma (a type of skin cancer). This is rare overall, but higher in patients with very large or multiple naevi.

What are the causes?

Birthmarks are the visible effect of changes that have occurred during a child’s development before birth. CMN are caused by a genetic mutation that occurs during development in the womb. When there are very large or multiple CMN, or problems with the brain, this is caused by a mutation occurring very early in development. This mutation has a very low chance of occurring again in the same family, and is not passed on to future generations. In other words it only causes CMN in one person.

How is it diagnosed?

CMN are diagnosed clinically – in other words by a specialist doctor looking at the birthmark(s). The associated problems in the brain need a magnetic resonance imaging (MRI) scan to be diagnosed.

How is it treated?

Most often no treatment is required. If the CMN is somewhere very visible, such as a large one on the face, then surgery may be available to remove it. This should be discussed with a paediatric dermatologist (children’s skin specialist).

Inheritance patterns and prenatal diagnosis

Inheritance patterns
CMNs are chance events and there is very little risk for further children

Prenatal diagnosis
It is very unlikely that the presence of a CMN could be identified by ultrasound scanning or any other method. Occasionally something is seen on the skin but not diagnosed as they are rare.

Is there support?

Caring Matters Now

Tel: 07786 458 883

Caring Matters Now is a Registered Charity in England and Wales No. 1120988. It provides information and support for people with Congenital Melanocytic Naevus (CMN) and their families. 

Group details last updated December 2014.

Information and support in the UK for congenital melanocytic naevi is also provided by the Birthmark Support Group (see entry Vascular Birthmarks).

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