Fetus In Fetu: 11 Fetoid Forms in a Single Fetus

Abstract

The term in was first coined by Johann Friedrich Meckel in the late 18th century to describe an encapsulated fetoid tumor within a fetus. Since then, fewer than 100 cases of fetus in fetu have been reported in the medical literature. The incidence of fetus in fetu is estimated to be approximately 1 per 500,000 births, 1 occurring relatively equally in male and female patients. 2 Debate exists as to whether fetus in fetu is a variant in the spectrum of monozygotic twinning or if it is simply a highly differentiated teratoma. The presence of a partial or whole vertebral column plus other appropriately situated axial or appendicular bones or organs constitutes the hallmark of fetus in fetu. 3 The conservative definition is the presence of a vertebral axis, which occurs in most cases. The more liberal definition was proposed by Gonzalez-Crussi, 4 who defined fetus in fetu as high organotypic development and presence of a vertebral axis with arrangement of tissues around this axis. It has been suggested that fetus in fetu is part of a spectrum of anomalous embryogenesis, including conjoined symmetric twins, parasitic fetuses, embryonic vestigial fetal inclusions, organized teratoma, and the twin reversed arterial perfusion sequence (also referred to as acardiac monster).