A congenital nevus is a mole that you’re born with. These moles can be large or small, have hair growing out of them, or change over time.

Congenital nevus (or nevi) is the medical term for a mole that you’re born with. They’re a very common type of birthmark. You might also hear them called congenital melanocytic nevi (CMN).

A congenital nevus looks like a round or oval-shaped patch of pigmented skin. It is usually raised. Congenial nevi can be either a single color or multi-colored. They can vary in size from a tiny spot to something that covers a large part of your body. In some cases, they might have hair growing out of them.

Your skin gets its color from pigment-producing cells called melanocytes. Nevi (moles) form when these cells group together in one place, rather than evenly distributed throughout your skin. In the case of congenital nevi, this process happens during the fetal stage.

A congenital nevus may become smaller or larger over time. It might also become darker, raised, and more bumpy and hairy, especially during puberty. In rare cases, it may disappear entirely.

Congenital nevi usually don’t cause any symptoms, but they’re occasionally itchy when they’re larger. The skin also might be a little more fragile and easily irritated than the surrounding skin.

There are several types of congenital nevi, depending on their size and appearance.

Large or giant

Nevi grow as your body grows. A nevus that will grow to an adult size of 8 inches or more across is considered a giant nevus.

On a newborn child, this means that a nevus that measures 2 inches across is considered a giant one. However, because the head grows somewhat less than the rest of the body, a nevus that measures 3 inches across on the head of a newborn is also classified as giant.

Giant nevi are relatively rare, occurring in roughly 1 out of 20,000 live births.

A doctor might classify a congenital nevus as large if it:

  • is larger than the palm of the child’s hand
  • is not removable by a single surgical cut
  • covers a large part of the head, legs, or arms

They may classify a congenital nevus as giant if it:

  • covers a very large part of the body
  • involves much of the torso
  • is accompanied by many smaller (satellite) nevi

Small and medium congenital nevi

A congenital nevus that measures less than 1.5 centimeters (cm) across (about 5/8 inch) is classified as small. These are fairly common, occurring in about 1 in every 100 newborn children.

A nevus that’s expected to grow to an adult size of 1.5 to 19.9 cm across (5/8 to 7 3/4 inches) is classified as medium. Medium nevi occur in about 1 in every 1,000 newborns.

Other types

Other types of congenital nevi include:

  • speckled lentiginous nevus, which has dark spots on a flat, tan background
  • satellite lesions, which are smaller moles, either surrounding the main nevus or located somewhere else on the body
  • tardive nevus, which is a nevus that appears after birth, usually before age 2, and grows slowly
  • garment nevus, which refers to a nevus either around the buttocks or on the entire arm or shoulder
  • halo nevus, which is a mole with light- or white-colored skin surrounding it

Researchers aren’t sure about the exact causes of congenital nevi. However, they do know that they start to grow between 5 and 24 weeks. The earlier they start growing, the larger they usually are at birth.

In most cases, congenital nevi don’t cause any physical problems and don’t require treatment. However, they can make some people self-conscious.

It’s hard to surgically remove congenital nevi, especially large and giant ones. These may require several cuts, stitches, or even skin replacement. All of this can result in scarring that some people find more bothersome than the mole itself.

Your doctor can give you a better idea of whether surgery will work based on the size and type of nevus.

Some alternatives to surgery include:

  • Dermabrasion. This treatment uses a wire brush or diamond wheel to remove layers of skin. While it won’t completely remove a congenital nevus, it can lighten its appearance. However, it can also leave scarring. Dermabrasion is most effective when done in the first six weeks of life.
  • Skin curettage. This involves scraping away the top layers of skin. Like dermabrasion it is best performed in the first six weeks of life.
  • Tangential excision. The top layers of skin are removed using a blade. Like other options, it won’t remove the nevus completely, and it may leave scarring. However, it can make the nevus less noticeable.
  • Chemical peels. These may help to improve the appearance of lighter-colored nevi. Phenol and trichloroacetic acid are common chemicals used in peels.

While most congenital nevi are harmless, they can occasionally become cancerous. Giant congenital nevi carry the highest risk. Keep in mind that surgery isn’t a guarantee against cancer. Fifty percent of melanomas found in people with giant congenital nevi occur elsewhere on the body. In addition, the estimated lifetime risk of melanoma for a person born with a giant nevus varies from 5 to 10 percent.

Medium and large nevi may also have a higher risk of becoming cancerous.

Anyone born with a large, giant, or even medium congenital nevus should get regular skin exams. Make sure to tell your doctor if you notice any of the following:

  • darkening of the nevus
  • lumpiness
  • increase in size
  • irregular shape
  • changes in color

Neurocutaneous melanocytosis is another possible complication of giant congenital nevi. This condition involves the presence of melanocytes in the brain and spinal cord. It affects an estimated 5 to 10 percent of people with giant congenital nevus. In many cases, it doesn’t have any symptoms, but it may occasionally cause:

  • headaches
  • vomiting
  • irritability
  • seizures
  • developmental issues

Congenital nevi are both common and usually harmless. However, there is a risk of complications, including skin cancer, in cases where a congenital nevus is larger than 2 or 3 inches. If the mole bothers you, talk to your doctor about which treatment options would work best for the size of your mole and your skin type.