This article was last reviewed and updated on June 13, 2019
by Robyn Ndikumana MD BScN and Allison Osmond, MD FRCPC
A congenital nevus is a non-cancerous type of tumour that develops in the skin at or near birth.
Another name for congenital nevus is mole.
A congenital nevus is made up of special cells called melanocytes which make a dark pigment called melanin.
Skin is made up of three layers: epidermis, dermis, and subcutaneous fat. The surface and the part you can see when you look at your skin is called the epidermis. The cells that make up the epidermis include: squamous cells, basal cells, melanocytes, Merkel cells, and cells of the immune system. The squamous cells in the epidermis produce a material called keratin which makes the skin waterproof and strong and protects us from toxins and injuries.
The dermis is directly below the epidermis. The dermis is separated from the epidermis by a thin layer of tissue called the basement membrane. The dermis contains blood vessels and nerves. Below the dermis is a layer of fat called subcutaneous adipose tissue.
What is a congenital nevus?
A congenital nevus is a common non-cancerous tumour made up of melanocytes. Melanocytes are small cells that produce melanin, a dark pigment that helps protect our skin from the sun’s ultraviolet rays.
The amount of melanin normally found in our skin determines our skin colour. People with very light skin produce very little melanin while people with very dark skin produce a lot of melanin. A congenital nevus looks dark because it is made up of melanocytes that are producing a lot of melanin.
Congenital nevi (nevi is the plural of nevus) are more common in people with light coloured skin. They 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. They can be found anywhere on the body, but the most common locations are the trunk and limbs.
Congenital nevi range in size from very small to very large. A congenital nevus is called a giant congenital nevus when it is larger than 20 cm. Many congenital nevi have an irregular shape and they can range in color from light brown to black. It is not uncommon to find a hair growing out of a congenital nevus.
While a congenital nevus is a non-cancerous skin tumor, they can change over time into a type of skin cancer called melanoma. You should let you doctor know if your nevus suddenly changes in size or colour or if it starts to bleed.
In general, when your doctor is looking at a nevus on your skin, they are looking for some basic features known as ABCDE.
A stands for asymmetry which means one side of the nevus looks different from the other.
B stands for for border. A nevus should have a smooth border.
C stands for color. You should talk to your doctor if your nevus changes colour or if it starts to show multiple colours.
D stands for diameter. This is the size of the nevus measured from side to side. You should talk to you doctor if your nevus suddenly starts to grow.
E stands for evolving or 'everything else'. This can include sudden changes in size or shape or a nevus that has started to bleed.
A nevus may be removed by your doctor if there are any changes in ABCDE, or for cosmetic reasons. Some nevi will grow back after being removed and this is known as recurrent or persistent nevus. If you see any colour in a scar after nevus is removed, you should see your family doctor.