Three-dimensional sonographic imaging of a highly developed fetus in fetu with spontaneous movement of the extremities.

fetus in fetu (FIF) is a rare anomaly of fetal development in which a pedunculated, fetiform mass develops inside a more mature fetus, 1,2 Minimal criteria for diagnosis include the presence of an axial skeleton or a fetus with metameric organization, skin coverage, encapsulation, and a 2-vessel cord. 3 In most cases that meet diagnostic criteria, the FIF is poorly developed and may be difficult to distinguish from a teratoma. The etiology of this condition is unknown. Although the diagnosis of an FIF has traditionally been made in infancy or childhood, there are an increasing number of diagnoses being made prenatally on the basis of sonography. 1,4-14 This report describes the prenatal diagnosis of an FIF with an extraordinarily high degree of differentiation using two-dimensional (2D) and three-dimensional (3D) sonographic imaging.