Teaching Case

A 22-year-old primigravida with a history of 5 months of amenorrhea was referred to our department for the second trimester anomaly scan. At the time of presentation, the patient had no complaints. The conception was spontaneous and there was no history of intake of fertility medication.

The antenatal ultrasound showed a fetus corresponding to 19 weeks gestation. There was a well-defined heterogeneous extra-axial vascular lesion measuring 49 x 35 millimetres within the fetal cranium surrounded by a membrane, displacing fetal brain to one side (Figure 1). One pole of the lesion showed peripheral oval calcification resembling a miniature fetal skull (Figure 2). Two pairs of isoechoic projections were seen arising from one side of the lesion resembling limb buds. Magnetic Resonance Imaging showed a heterogeneous lesion with a thin surrounding membrane (Figure 3). Conventional radiograph (Figure 4) and Computed Tomography scan (Figure 5) were performed after the demise of the aborted fetus (Figure 6) which revealed calcified areas resembling miniature fetal cranium and vertebrae. The excised intracranial mass measured 53 x 37 millimetres and resembled a fetus with two pairs of limb buds, vertebral elements and cranium enclosed in a sac (Figure 7) confirming the diagnosis.

Fetus-in-fetu, a term quoted by Lewis is a very rare entity. It was first described by Meckel in 1800. Fetus-in-fetu is presumed to result from the unequal division of totipotent inner cell mass of the developing blastocyst leading to the inclusion of a much smaller cell mass within a maturing embryo. This process leads to the occurrence of a diamniotic monochorionic twin within the body of a normally developing twin fetus [1].

Some researchers consider fetus-in-fetu to be one spectrum of malignant teratoma. The incidence is about 1 in 500,000 births. Less than 200 cases have been reported in the literature, of which only 2 cases were intracranial in location [2, 3].

Fetus-in-fetu most commonly occurs in the abdomen of its host twin as a fetus-like, calcified mass. Retroperitoneum is the most common site of occurrence. Cranial cavity, pelvis, scrotal sac, mesentery, right iliac fossa and oral cavity are other rare locations [4].

Fetus-in-fetu is usually seen as a well-defined complex mass often enclosed within an amnion-like membranous capsule. Long bones and fat may be identified on plain films, but are better demonstrated with Computed Tomography or Magnetic Resonance Imaging studies. Computed Tomography scan can also demonstrate the vascular pedicle supplying the mass. On ultrasound, it is seen as a mass with heterogeneous complex echo pattern due to the presence of fluid, soft tissue and calcification within it [4].

Fetus-in-fetu can be differentiated from a teratoma by the presence of a highly developed state of organogenesis or by the presence of a vertebral axis. Teratoma is an accumulation of pluripotent cells with neither organogenesis nor vertebral segmentation [5].

Teratomas are most commonly found in the lower retroperitoneum, ovaries and sacrococcygeal region whereas upper retroperitoneum is the most common site for a fetus-in-fetu [6, 7].

Malignant degeneration associated with fetus-in-fetu is exceedingly rare, with only one case reported so far [8]. Complete excision with meticulous dissection is curative and allows confirmation of diagnosis in postnatally diagnosed cases [9].