Congenital melanocytic naevus (CMN)

What is a congenital melanocytic naevus (CMN)?

These aren't vascular birthmarks, but are areas with large numbers of melanocytes – cells which produce melanin, a pigment that gives your skin its colour. When grouped together, melanocytes make the skin darker.

The colour of this type of birthmark varies from mid-brown to black, depending on natural skin colour. They may be raised or have hairs growing out of them.

There is very low risk of malignant melanoma, a type of cancer, developing within small or medium-sized CMNs; but the risk increases in large CMNs, especially across the spine.

Many CMNs will lighten over the first few years of life although the degree of lightening will vary between individuals.

Where do they occur?

CMNs are most often found on the back, or head and neck but can occur anywhere on the body including the hands and feet.

How common are they?

About 1 in 100 babies is born with a CMN, although most are small and less than 2.5cm (1 inch) across. Larger and multiple CMNs are more rare and only occur in approximately 1 in 20,000 births.

Treatment

Many CMNs do not need specific treatment unless there is risk of melanoma (a type of cancer), the CMN either grows or becomes darker or lumpier. Surgical removal is an option depending on the location, but it can leave scarring.

This site does not provide medical advice and is not a substitute for medical or professional care, and if you see a birthmark growing or changing significantly, see a specialist.