Fetus-in-fetu. Imaging and pathology.

Fetus-in-fetu (FIF), also known as endoparasitic twin, is a form of asymmetric fetal duplication in which the abnormal developing embryo parasitizes the normal co-twin by attaching internally. Here, we report a case of FIF presented as an intra-abdominal cystic mass, which was first detected during an antenatal ultrasound examination of a 32-year-old Saudi mother. At 34 weeks and 4 days of gestation, she had spontaneous labor and delivered a baby boy.