Fetus-in-fetu (FIF) is an extremely rare developmental abnormality with an incidence of 1 in 500 000 births. it was first reported in 1808 by George Young. Since then, less than 200 cases have been reported worldwide.
FIF is characterized by the incorporation of one or more partially-developed fetuses into the body of an otherwise normally developed fetus and most commonly presents with a mass in the abdomen.
In this case, a 5 days old infant was referred from a neighbouring hospital to Aster MIMS Kottakkal, Kerala, India for scanning of an abdominal mass. During the ultrasound, a cystic structure with calcified limb-like structures within it was found. It resembled a fetal limb, near to the liver and kidneys of the neonate. Further, a 3D & 4D scan was performed, and the diagnosis of Fetus in fetu was confirmed. The malformed Fetus was excised in Kozhikode Medical College on the 45th day after birth. Postoperatively the patient recovered well without any complications. Sonographic reports of such a case are nominal. However, the 3D images of Fetus in fetu limb has not been acquired anywhere else.
Multiple theories are proposed regarding the embryogenesis of Fetus-in-fetu (FIF). Most recent literature review suggests that it is a diamniotic, monochorionic, monozygotic twin in the body of the host twin after anastomosis of the vitelline circulation. Other theories include demised multiple pregnancies and a variant of teratoma.