Intra-abdominal retroperitoneal fetus in fetu: A case report.

Fetus in fetu (FIF) is a rare abnormality where a vertebrate parasitic fetus develops inside the body of another normally developing fetus. It is distinct from teratomas, tumors composed of cells from multiple germ layers and have malignant potential. Symptoms of FIF arise from the mass effect, causing abdominal distension, feeding difficulties, and pressure effects on organs.