Background: Fetus-In-Fetu (FIF) is a rare congenital malformation, which consists of a fetus mixed with a distinct tissue that is from the other fetus of twin. FIF is defined as a mass containing a vertebral axis often associated with other organs or limbs around this central axis.
Case report: The patient was a two day old girl infant who due to a mass in a sacrum area measuring 8.5 x 12.5 cm with soft consistency containing a normal skin color fluid with blood streaks and hemorrhagic and bruise colored patches since birt, was admitted at the Taleghani children's Pediatric Center of Gorgan. The results of the mother's ultrasound in the fourth month of pregnancy had identified the cyst for the fetus. In the primary results of imaging, the first the diagnosis was type 2 sacrocoxigeal teratoma. However, results of post-birth graphy confirmed the presence of fetal bone structure in the infants's sacrum, so FIF was diagnosed for the infant.
Conclusion: In the present study, FIF has been seen in sacrum, which is a very rare case in FIF.