Foetus-in-foetu versus Well-differentiated Sacrococcygeal Teratoma: A Pathological Dilemma

Rationale: Foetus-in-foetu (FIF) is a rare congenital anomaly seen in young infants or at birth. It is a parasitic twin in a diamniotic, monozygotic twin. It needs to be differentiated from teratoma, as the latter has malignant potential. Patient Concern: Here, we present a case of an 11-day-old infant presenting with mass abdomen since birth. Microscopic examination showed skin with adnexal structures, neural, respiratory, gastrointestinal tissue, cartilage, bone and ocular structures also identified, which showed normal histology with the same degree of maturation. Diagnosis: A diagnosis of FIF was reported based on the radiological which was confirmed on histopathology. Outcome: The patient is doing well post-surgery. Lessons: This case highlights the significance of radiological, clinical, intraoperative and pathological features that help distinguish the two conditions.