Fetus in fetu

Fetus in fetu is a rare congenital anomaly in which there is an incorporation of one or more partially developed fetus into the body of an otherwise normally formed twin. It is a malformed parasitic twin included in the host twin due to the unequal division of totipotent cells of a blastocyst.

FIF occurs due to abnormal embryogenesis.

Fetus in fetu can be differentiated from a teratoma in that it has an axial skeleton with organized organs and limbs.

From genetics its thought to occur in monozygotic, monochorionic diamniotic twin gestation.

It occurs when one twin gets absorbed by the other during ventral folding of the trilaminar embryonic sac

FIF does not contain cancerous potential. As a result, an insult to the host twin may occur because of the mass effect on the internal organs.

Fetus in fetu is often overlooked in the differential diagnosis of an abdominal mass. Unlike teratomas, a fetus in fetu is a benign disorder.

FIF presents as a lump in the abdomen in 70% of the cases. Most commonly located in the retroperitoneal space. It can also be found in the sacrococcygeal region, intracranial or in the thorax.

Malignant transformation of the fetus in fetu is rare.

Theories

There are two theories that try to explain its development;

One theory is that the mass begins as a normal fetus but becomes enveloped inside its twin. The other theory is that the mass is a highly developed teratoma.

Teratoma theory (Teratoma)

Fetus in fetu may be a very highly differentiated form of a dermoid cyst, itself a highly differentiated form of mature teratoma.

Parasitic twin theory (Parasitic twin)

Fetus in fetu may be a parasitic twin fetus growing within its host twin.

Very early in a monozygotic twin pregnancy, in which both fetuses share a common placenta, one fetus wraps around and envelops the other. The enveloped twin becomes a parasite, in that its survival depends on the survival of the host twin, by drawing on the host twin's blood supply.

The parasitic twin is anencephalic and lacks some internal organs, and as such is unable to survive on its own.

As the host twin has to "feed" the enveloped twin from the nutrients received over a single umbilical cord, they usually die before birth.

It should be differentiated from a teratoma because of the teratoma’s malignant potential.

Preoperative diagnosis is based on radiologic findings. The treatment of fetus in fetu is operative to relieve obstruction, prevent further compression and possible complications.

Complete excision allows confirmation of the diagnosis and lowers the risk of recurrence.