Introduction Fetus-in-fetu (FIF) is a rare congenital anomaly with an incidence of 1:500,000 births that is characterized by the incorporation of one or more partially-developed fetuses into the body of an otherwise normally developed fetus. First described in 1800, FIF was initially thought to represent a mature teratoma. It has since been defined as a separate entity, distinguished by the presence of an axial skeleton with organized limbs and organs. Genetic studies suggest that this phenomenon occurs in monozygotic monochorionic diamniotic twin gestations when one twin is absorbed by the other during ventral folding of the trilaminar embryonic sac. Unlike teratomas, FIF does not have malignant potential and the vast majority of cases are discovered incidentally. Insults to the host twin can occur, however, largely as a result of mass effect on vital organs. Case Report We present the case of a preterm female infant with an abdominal mass and ascites initially found on prenatal ultrasound that was later diagnosed on autopsy as FIF. Repeat ultrasound identified a complex cystic mass within the liver containing multiple bony structures. The patient’s mother was admitted to UNC Hospitals at 26 weeks gestation with hypertension, proteinuria and oliguria. On hospital day 2, the patient’s mother developed vision changes, prompting an emergent cesarean section. At the time of delivery the patient’s abdomen was markedly distended and contained a palpable mass with bony prominences. The infant was intubated for poor respiratory effort and paracentesis was performed, yielding 325 mLs of ascitic fluid. The patient’s clinical status quickly deteriorated with significant metabolic acidosis, poor ventilation, anemia and electrolyte disturbances. Parents elected to withdraw care and the patient expired at 12 hours of life. Postmortem MRI demonstrated a rudimentary fetus within the peritoneal cavity with a partially calcified skeleton, encephalocele, gastroschisis, two upper extremities and four lower extremities. Three teratomas were also identified, the largest of which bifurcated the native liver. Paracentesis aspirate was later identified as amniotic fluid. Discussion Autopsy for this patient revealed mass effect on the thoracic cavity with disruption of the left diaphragm. In combination with the blood flow and oxygen demand needed to support the retained twin and teratomas, this likely contributed to this patient’s abrupt decompensation. As the majority of patients with FIF have a largely benign course, this case highlights the potential of this condition to result in significant complications. This case is also the first to confirm the presence of amniotic fluid in FIF. Conclusion With less than 200 reported cases, FIF remains exceedingly rare. Knowledge of this condition is important in order to appropriately differentiate it from malignant processes like teratomas, but as demonstrated by this case, FIF can also result in significant morbidity and mortality.

Host Twin Postmortem

Postmortem image of the host twin, demonstrating significant abdominal distention resulting from the parasitic twin and three intraperitoneal teratomas.

Host Twin Postmortem

Postmortem image of the host twin, demonstrating significant abdominal distention resulting from the parasitic twin and three intraperitoneal teratomas.

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Three-Dimensional Skeletal Reconstruction

Computer Tomography scans were obtained prior to autopsy to better characterize the anatomy of the parasitic twin. A rudimentary skull with a large anterior defect, multiple malformed vertebral bodies, ribs, bilateral dysplastic right upper extremities as well as four lower extremities of various degrees of development.

Three-Dimensional Skeletal Reconstruction

Computer Tomography scans were obtained prior to autopsy to better characterize the anatomy of the parasitic twin. A rudimentary skull with a large anterior defect, multiple malformed vertebral bodies, ribs, bilateral dysplastic right upper extremities as well as four lower extremities of various degrees of development.

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