Section Editor: Allison Osmond MD FRCPC
November 14, 2024
A congenital nevus is a common, noncancerous (benign) skin growth made up of cells called melanocytes. Melanocytes are the cells in the skin that make melanin, the pigment that gives skin its color. These growths are called “congenital” because they are usually present at birth or appear within the first year of life. Another name for this kind of growth is a mole, the everyday word for any growth made of melanocytes.
Congenital nevi (the plural of nevus) are more common in people with lighter skin and can appear anywhere on the body, most often on the trunk and limbs. A congenital nevus is harmless, and this article explains what it is, what the findings in your (or your child’s) pathology report mean, and why it is considered safe.
What causes a congenital nevus?
A congenital nevus forms when melanocytes overgrow in one area of the skin during a baby’s development before birth. This overgrowth is caused by a change (called a mutation) in a gene that controls cell growth, most often NRAS, especially in larger nevi. This change happens by chance in the developing skin cells. It is not inherited from a parent and is not caused by anything the mother did during pregnancy.
How common are congenital nevi?
Congenital nevi are relatively common. About 1 in 100 newborns has a small congenital nevus. Medium-sized nevi are less common, and giant congenital nevi are rare. The size of a congenital nevus is usually described by how large it is expected to be in adulthood: small (less than 1.5 cm), medium (1.5 to 20 cm), or large to giant (more than 20 cm).
What are the symptoms of a congenital nevus?
A congenital nevus usually causes no symptoms. Its appearance can vary widely, from very small to very large. Most are round to oval or somewhat irregular in shape, with colors ranging from light brown to black. It is common and normal for hair to grow from a congenital nevus, and this does not indicate any health problem. A congenital nevus normally grows in proportion with the child and stays stable in color and shape. Any change in size, shape, or color that seems out of proportion, or new itching or bleeding, should be checked by a doctor.
How is the diagnosis made?
A congenital nevus can often be recognized by a doctor based on its appearance and its presence from early life. When a nevus is removed, because it is changing, is being checked to rule out melanoma, or for cosmetic reasons, the tissue is examined under the microscope by a pathologist after a biopsy or excision.
Under the microscope, a congenital nevus is composed of melanocytes, which often contain dark melanin pigment, that grow mainly within the dermis, the layer of skin beneath the surface. A characteristic feature is that melanocytes cluster around normal skin structures such as hair follicles, oil (sebaceous) glands, nerves, and blood vessels, a pattern that helps the pathologist distinguish a congenital nevus from other moles. In larger nevi, the melanocytes may extend deeper into the fatty layer beneath the dermis (the subcutaneous tissue). These features, together with the orderly, mature appearance of the cells, tell the pathologist that the growth is benign rather than melanoma (a cancer made of melanocytes).
Can a congenital nevus turn into melanoma over time?
It is uncommon for a congenital nevus to turn into melanoma. For small and medium congenital nevi, the risk is very low. The risk is higher for giant congenital nevi (larger than 20 cm), though most of these do not develop into melanoma. Watching the nevus for changes in size, shape, or color, or for a new lump, itching, or bleeding, is the best way to catch any problem early, especially for larger nevi.
What happens after this diagnosis?
Most congenital nevi need no treatment beyond regular observation. When a congenital nevus is removed, it may be for cosmetic reasons, because it is irritated, because it is in a hard-to-monitor location over time, or because of a change that needs to be examined under the microscope. Removal is a decision made with your doctor, based on the size and location of the nevus and your personal preference.
Large and giant congenital nevi are usually managed by a team that may include a dermatologist and a plastic surgeon. These nevi are sometimes removed in stages, both to improve appearance and to reduce the risk of melanoma, although removal cannot always remove every melanocyte because some lie deep in the skin. In infants with a large or giant nevus, particularly when there are many smaller “satellite” nevi, a doctor may recommend an MRI of the brain and spine to check for a condition called neurocutaneous melanosis, in which melanocytes are also present in the central nervous system. For all congenital nevi, regular skin checks and protection from sun damage are important parts of long-term care.
Questions to ask your doctor
- What size category is this congenital nevus (small, medium, or large to giant)?
- Was the whole nevus removed, or could some of it have been left behind?
- What is the risk of this turning into melanoma?
- Does the size or location mean it needs closer monitoring?
- For a large nevus, should we see a dermatologist or plastic surgeon?
- For a large nevus, is an MRI needed to check for neurocutaneous melanosis?
- Would removal be recommended, and if so, why?
- How often should the nevus be checked?
- What changes should prompt me to call you?
- What can we do to protect the skin from sun damage?
Related articles
We are proud to partner with:
