Congenital nevus

What is congenital nevus?

A congenital nevus (con·​gen·​i·​tal - ne·​vus) is strictly called congenital melanocytic nevi (me·​la·​no·​cyt·​ic - nevi). They are brown or black moles which are present at birth, or which develop in the first month or so of life (brown birthmarks).

• Giant congenital nevi are often found on the buttocks when they are known as "bathing trunk" nevi. A speckled lentiginous nevus (also known as nevus spilus ) has dark spots scattered on a flat tan background. 
• A café-au-lait macule is a flat tan mark, usually oval in shape: the tendency for these is inherited.

Congenital nevi are harmless overgrowths of the cells in the skin responsible for normal skin color. In some cases, there is also overgrowth of the hair-forming cells. About one baby in 100 has a small or medium sized congenital nevus, so they are quite common.

Very large, "giant" or "bathing trunk" nevi are very rare. We do not know why they develop. Most congenital nevi grow with the child, and generally they are proportionally smaller and less obvious with time. However, sometimes the mole becomes darker especially at puberty. It may also become more hairy.

• Melanoma, a cancerous mole, sometimes develops within congenital nevi. The risk of this happening in a small or medium-sized mole is very small (under 1%) and melanoma never arises from café-au-lait macules. Melanoma is more likely in the giant nevi (perhaps about 5% over a lifetime) especially in those that lie across the spine; the cancer can start in the skin, in pigment cells in the spine or even in the brain. It is then very difficult to detect and treat.