Intracranial fetus-in-fetu with numerous fully developed organs.

Fetus in fetu (FIF) is an extremely rare anomaly featuring a monozygotic, diamniotic, parasitic twin, enclosed within its host twin. It is characterized by the presence of vertebrae and well-developed organs in a fetiform mass. Only 18 cases of intracranial FIF have been published. Of them, only five cases were prenatally detected. This study prenatally demonstrated triplet FIFs at 31 weeks within amniotic-like sac in the fetal skull, consisting of multiple well-defined organs. The FIF attached to the host twin via body stalk containing a single main feeding artery and vein, representing umbilical vessels. Surgical removal was performed at the age of two months. Pathological examination showed the triplet FIF, consisting of numerous well-developed organs (musculocutaneous-skeletal, nervous, respiratory, gastrointestinal systems etc.), with soft tissue/skin coverings, but no vertebral body was seen. Molecular genetic analysis revealed identical genetic mapping among the three FIFs and the host. This case provides strong evidence against Willis's hypothesis but supports Spencer's theory of abnormal twinning.