What type of congenital nevus is usually present?

NCM occurs in 7.5% of patients with giant truncal nevi and approximately 70% of patients with medium-sized congenital nevi and multiple progressive satellite lesions.

Bett BJ: Large or multiple congenital melanocytic nevi: occurrence of neurocutaneous melanocytosis, J Am Acad Dermatol 54:767–777, 2006.