The Silent Passenger: The Medical Mystery of Fetus in Fetu

Fetus-in-fetu is an exceptionally rare condition that arises due to abnormal embryonic development in a monochorionic diamniotic twin pregnancy. In this scenario, a non-viable fetus becomes encapsulated within a normally developing sibling. The condition has an estimated occurrence of 1 in 500,000 live births ⁽¹⁾ and appears to be twice as common in males as in females. ⁽²⁾ There are two main theories to explain this rare anomaly: ⁽³⁾

• One suggests it results from abnormal embryogenesis during the development of identical twins, where one malformed twin becomes enclosed within the other.
• Another theory considers it to be a highly organized form of teratoma.

In most cases, the enclosed fetus is found in the abdominal area, particularly the retroperitoneal space (in about 80% of cases). ⁽⁴⁾ Less common locations include the cranial cavity, mediastinum, scrotum, and pelvis. The patient usually present with abdominal mass, vomiting or feeding difficulties, distended abdomen, pain or discomfort.

Case:

During the Antenatal scan done at 28 weeks of gestation, well defined cystic area was noted in the right hypochondriac region of the fetus with partially ossified soft tissue structure within it. The baby was born at 36 weeks of life by normal vaginal delivery. The peripartum period was uneventful. In the postnatal period, the baby was subjected to X ray, Ultrasonography and CECT scan. The spectrum of imaging investigations revealed Fetus in Fetu. On local examination, there was fullness in the right hypochondrium with a round intra-abdominal, non-tender mass of variable consistency.

On ultrasonography performed on day 1 of life: