Fetus-in-fetu: a rare condition that requires common rules for its definition

Excerpt

Fetus-in-fetu (FIF) is a rare clinical condition that was firstly described by Meckel in 1800, and less than 200 cases have been reported so far [1]. Although adult cases were already described, FIF is typically a pediatric condition managed after birth in children younger than 18 months of age, with symptoms and signs primarily related to its mass effect [2]. Nevertheless, the development of obstetric ultrasonography allowed to anticipate the diagnosis as early as 21 weeks of gestation. Although the definitive diagnosis is performed after surgery, ultrasonography, radiography, computed-tomography and magnetic resonance imaging play a key role in the prenatal diagnosis of FIF [3, 4]. This rare condition is considered part of monozygotic twin pregnancies spectrum, and it is supposed to derive from an unequal division of the totipotent inner cell mass of a developing blastocyst. In details, the small cell mass develops within the developing embryo in the vestigial remnant, representing the co-twin in a diamniotic monochorionic pregnancy incorporated within the normal co-twin’s body [3]. We take the opportunity to describe a case recently observed in our practice, aiming to discuss different definitions of FIF based on anatomical and embryological criteria. …