A congenital pigmented or melanocytic nevus is a dark colored, often hairy patch of skin that is present at birth.

Smaller in infants and children, they usually continue to grow along with the child. A giant pigmented nevus is larger than 8 inches once it stops growing.

Bathing trunk nevi are thought to be caused by defects or problems that occur as a baby grows in the womb. However, in some families bathing trunk nevus may be inherited.

Bathing trunk nevus may occur with:

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

Smaller congenital pigmented or melanocytic nevi can be common in children and are less dangerous. However, larger or giant nevi are fairly rare.

• Brown to bluish-black color
• May contain hair
• Skin surface may be smooth to irregular or wart-like
• Borders of these lesions are regular or uneven
• Small, satellite areas (maybe)

They are commonly found on the upper or lower parts of the back or the abdomen. They may also be found on the arms, legs and elsewhere on the body, including the palms, soles, and even the mucous membranes or mouth.

All birthmarks should be evaluated by your health care provider. A biopsy of suspicious areas may be obtained for examination to determine if the cells have become cancerous. An MRI of the brain might be performed if the lesion is over the spine.

Treatment consists of frequent examination to check for skin cancers and, when possible, surgery to remove the nevus. Skin grafting is done when necessary. Larger nevi may need to be removed in several stages.

Lasers and dermabrasion (link) can also be used to improve the appearance. However, using these techniques may not remove the entire birthmark, and may make it harder to diagnose skin cancer (melanoma).

Psychological treatment can help with the emotional impact of having a disfiguring disorder.

Skin cancer (such as malignant melanoma and other types) may develop in up to 15% (1 out of 6) of people with larger or giant nevi, often in childhood. The risk is higher for larger or giant congenital nevi located on the back or abdomen.

• Depression and other emotional problems (due to appearance)
• Skin cancer (melanoma)

Rarely, bathing trunk nevi occur with a condition that causes a growth of pigment-producing cells in the head (leptomeningeal melanocytosis). Complications include:

• Hydrocephalus
• Motor abnormalities
• Seizures

This condition is usually diagnosed at birth. Call for an appointment with your health care provider (or mention it during a well-baby exam) if your child has a large pigmented area anywhere on the skin.

Bett BJ. Large or multiple congential melanocytic nevi: Occurence of neurocutaneous melanocytosis in 1008 persons. J Am Acad Dermatol. 2006 May;54(5):767-777.

James WD, Berger TG, Elston DM, eds. Melanocytic Nevi and Neoplasms. Andrews' Disease of the Skin: Clinical Dermatology. 10th ed. Philadelphia, Pa: Saunders Elsevier; 2005:chap 30.