Congenital nevus is a rare dermatological lesion that is present at birth on the acral volar regions of the body and has an unsightly appearance. It is called a giant congenital melanocytic nevus when it attains a diameter of 20 cm. Confirmation of the diagnosis requires a histological evaluation of a biopsy specimen.
The incidence of congenital nevus is approximately 1 in 100 live births   and it may be observed by parents of affected infants/ children or may be detected in adolescence or even in adulthood. It can occur on any part of the body and is defined as a "giant melanocytic nevus" when it reaches a minimum diameter of 20 cm .
Congenital nevus in children and adolescents appears as a skin lesion with perifollicular hypopigmentation, irregular edges with projections or variegated pigmentation within the nevus . Compared to its acquired version, a congenital melanocytic nevus is large, asymmetrical and comma shaped and is located on the flexor surfaces of the extremities . In a majority of the cases, it is asymptomatic. It may be associated with infantile hemangiomas .
A giant congenital melanocytic nevus is also rare but is clinically important as it can be associated with malignant melanoma and neurocutaneous melanosis. In addition, it can cause a psychological impact on the patients and their families due to its ungainly appearance   with consequent disfigurement, especially when it is present on the face . It is a brown colored skin lesion with well-defined edges and hypertrichosis . It usually occurs on the torso but can also be found on the extremities and head    and often involves multiple segments of the body. It is mostly asymptomatic but patients may show the signs of pruritus.
A congenital nevus can be identified as such based on either history, medical records or photographic evidence of its presence since birth. The gold standard test, however, is a biopsy of the suspected lesion.
Magnetic resonance imaging (MRI) is indicated, especially if a congenital nevus is detected in a posterior region in an infant less than 4 months of age or if the infant has multiple satellite lesions  as it could be associated with neurocutaneous melanosis. MRI and scintigraphy can also help to detect rare cases of congenital nevi involving the bones or the maxillary sinus.
Dermoscopy is an excellent tool for evaluating the nevus and will demonstrate globular or reticular or a mixed pattern. The patterns vary with age and location - young children often have a globular pattern while those above 12 years of age have a reticular pattern .
Histological studies are essential due to the risk of malignant melanoma. A congenital nevus can be differentiated from an acquired nevus due to the following characteristics that are seen under a microscope   :