A congenital pigmented or melanocytic nevus is a dark-colored, often hairy patch of skin. A congenital nevus is present at birth or appears in the first year of life. 

A giant congenital nevus is smaller in infants and children, but it usually continues to grow as the child grows. A giant pigmented nevus is larger than 8 inches once it stops growing.

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

These marks are thought to be caused by problems that develop as a baby grows in the womb. In some families bathing trunk nevi may be inherited.

The condition may occur with:

• Growth of fatty tissue cells (lipomas)
• Neurofibromatosis
• Other nevi
• Spina bifida

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 dark-colored patch with any of the following:

• Brown to bluish-black color
• Hair
• Regular or uneven borders
• Small satellite areas (maybe)
• 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
• Mouth
• Mucus membranes
• Palms or soles

You should have all birthmarks looked at by a health care provider. A skin biopsy may needed to check for cancer cells. 

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

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

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

Surgery to remove the nevus is be done if possible. Skin grafting is also done when needed. Larger nevi may need to be removed in several stages.

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

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

Skin cancer may develop in about 1 in 6 people with large or giant nevi. The cancer risk is higher for nevi located on the back or abdomen. 

• 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 doctor if your child has a large pigmented area anywhere on the skin.

Congenital melanocytic nevi. In: Habif TP, ed. Clinical Dermatology. 5th ed. Philadelphia, Pa: Saunders Elsevier; 2009:pp 850-851.
Melanocytic Nevi and Neoplasms. In: James WD, Berger TG, Elston DM, eds. Andrews' Diseases of the Skin: Clinical Dermatology. 11th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 30.