Congenital nevus

by Robyn Ndikumana MD BScN and Allison Osmond, MD FRCPC
March 9, 2023

What is a congenital nevus?

A congenital nevus is a common non-cancerous type of skin tumour made up of melanocytes. Congenital nevi (nevi is the plural of nevus) are more common in people with light-coloured skin. They can be found anywhere on the body, but the most common locations are the trunk and limbs. The tumour is called ‘congenital’ because it forms before birth or during the first year of life. Another name for this type of tumour is a mole. Mole is a common term used to describe any kind of growth made up of melanocytes.

Is a congenital nevus a type of cancer?

No. A congenital nevus is a non-cancerous growth.

Can a congenital nevus turn into melanoma over time?

It is very rare for a congenital nevus to turn into melanoma over time.

How common are congenital nevi?

About 1% of newborns have small congenital nevi. Large nevi (those measuring over 1.5 cm) are much less common.

What does a congenital nevus look like without a microscope?

Congenital nevi range in size from very small to very large. A giant nevus is larger than 20 cm. Many nevi have an irregular shape and they can range in colour from light brown to black. It is not uncommon to find hair growing out of a congenital nevus.

How does a pathologist make the diagnosis of congenital nevus?

This diagnosis is usually made after the tumour has been surgically removed and sent to a pathologist for examination. When examined under the microscope, the tumour is made up of melanocytes. Dark melanin pigment is often seen throughout the tumour. The tumour cells grow in the layer of skin just below the surface called the dermis. As they grow, they can wrap around different normal structures like hair follicles, sebaceous glands, nerves, and blood vessels. Sometimes they can grow into the fat (subcutaneous tissue) below the dermis.