Fetus-in-fetu: mimicking teratoma on antenatal ultrasound

Fetus-in-fetu is a rare congenital anomaly that occur secondary to abnormal embryogenesis in a diamniotic monochorionic pregnancy. Its diagnosis can be accurately made by imaging ultrasonography, radiography, computed tomography, or magnetic resonance imaging. Differential diagnosis is an important issue because FIF, teratoma and cystic meconium peritonitis are very different in terms of their respective disease courses.

This is an interesting rare case of a 22-year-old pregnancy woman, presented for a routine antenatal ultrasound. The diagnosis of a fetus-in-fetu was suspected, complete surgical excision of the lesion was performed and the diagnosis was histopathologically confirmed.

We describe also the common characteristic of FIF as revealed by prenatal and postnatal US, postnatal MRI, and the operative findings.