These marks are thought to be caused by a localized increase in melanocytes as a baby grows in the womb. Melanocytes are the skin cells that produce melanin, which gives skin its color. A nevus has an increased amount of melanocytes.

The condition is thought to be caused by a variant gene.

The condition may occur with:

• Other nevi (moles)
• Involvement of the membranes of the brain and spinal cord when the nevus affects a very large area

Smaller congenital pigmented or melanocytic nevi are common in children and do not cause problems most of the time. Larger or giant nevi are rare.

A nevus will appear as a dark-colored patch with any of the following:

• Brown to bluish-black color
• Hair
• Regular or uneven borders
• Smaller affected areas near the larger nevus (sometimes)
• Smooth, irregular, or wart-like skin surface

Nevi are commonly found on the upper or lower parts of the back or the abdomen. They may also be found on the:

• Arms
• Legs
• Mucus membranes
• Palms or soles

You should have all birthmarks looked at by your health care provider. A skin biopsy may be needed to confirm the diagnosis or check for cancer cells.

An MRI of the brain or spine might be done if the nevus is over the spine. A giant nevus on the spine may be linked to brain or spinal cord problems.

Your provider will measure the dark skin area every year and may take pictures to check if the spot is getting larger.

You will need to have regular exams to check for skin cancer.

Surgery to remove the nevus can be done for cosmetic reasons or if your provider thinks it may become skin cancer. Skin grafting is also done when needed. Larger nevi may need to be removed in several stages.

Lasers and dermabrasion (rubbing them off) can also be used to improve appearance. These treatments may not remove the whole birthmark, so it may be harder to detect skin cancer (melanoma). Talk to your provider about the pros and cons of surgery for you.

Treatment may be helpful if the birthmark causes emotional problems because of how it looks.

Skin cancer may develop in some people with large or giant nevi. The cancer risk is higher for nevi that are larger in size. However, it isn't known if removing the nevus reduces that risk.

Having a giant nevus may lead to:

• Depression and other emotional problems if the nevi affect appearance
• Skin cancer (melanoma)

This condition is usually diagnosed at birth. Talk to your child's provider if your child has a large, pigmented area anywhere on their skin.

Congenital giant pigmented nevus; Giant hairy nevus; Giant pigmented nevus; Bathing trunk nevus; Congenital melanocytic nevus - large

Dinulos JGH. Nevi and malignant melanoma. In: Dinulos JGH, ed. Habif's Clinical Dermatology. 7th ed. Philadelphia, PA: Elsevier; 2021:chap 22.

Patterson JW. Lentigines, nevi, and melanomas. In: Patterson JW, ed. Weedon's Skin Pathology. 5th ed. Philadelphia, PA: Elsevier; 2021:chap 33.

Updated by: Elika Hoss, MD, Assistant Professor of Dermatology, Mayo Clinic, Scottsdale, AZ. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.