Fetus in fetu: a rare cause of a neonatal mass.

Fetus in fetu is an unusual digression from the normal twinning process which often presents as a mass in neonates. Approximately 80 reports have been documented in the literature to date. The increasing use of routine obstetric ultrasound has identified more cases during the antenatal period. This allows more time for both the neonatology team and the parents to develop a coordinated treatment plan for the anticipated child. We report a case of a fetus in fetu which was first seen as a complex cystic mass during antenatal ultrasound at 21 weeks and was confirmed by subsequent imaging and surgical treatment shortly after birth.