Congenital Fibromatosis Colli in a Newborn: A Rare Neck Mass Mimicking a Tumor: Case Report

Background: Fibromatosis colli is a rare benign condition in neonates characterized by the enlargement of the sternocleidomastoid muscle (SCM), typically following birth trauma. It can raise concern due to its presentation as a cervical mass. Case Presentation: We report the case of a 5-day-old male newborn, born to a primigravida mother after a difficult delivery. A firm, non-tender, left-sided neck swelling was noted from birth. Cervical ultrasound revealed a fusiform enlargement of the left SCM with preserved fibrillar structure, consistent with fibromatosis colli. Conclusion: Fibromatosis colli is a self-limiting condition and should be considered in neonates presenting with lateral neck swelling, particularly after complicated deliveries. Ultrasound remains the diagnostic tool of choice. Early physiotherapy can prevent complications such as congenital torticollis.