We report a case of prenatally diagnosed Fetus-in-fetu (FIF) residing in the left retro-peritoneum in a 2-week-old neonate which was also associated with ipsilateral testicular atrophy. A comparison of features differentiating FIF from a retroperitoneal teratoma, and various theories of origin of FIF are described. The causal relationship of ipsilateral atrophic testis with FIF in this case is ...

Fetus-n-fetu is a fascinating diagnosis explaining the development of ‘baby within a baby’. It results out of a monozygotic diamniotic twin pregnancy in which a twin is absorbed into the body of its sibling. The surviving twin often presents with abdominal mass and the diagnosis is only confirmed by surgical exploration. It is often mimics retroperitoneal teratoma on imaging and poses a challen...