Acardiac twin presenting as fetus amorphous with an attenuated umbilical cord.

Acardiac anomaly sequence is a rare malformation cluster occurring in the setting of monozygotic monochorionic twin pregnancies. In addition to an absent heart (acardia), variable degrees of somatic developmental disruption are present. We describe an extreme example of what we believe to be acardiac twinning, with almost complete absence of gross tissue organization but recognizable microscopic evidence of body-axis establishment and organ formation. The case is also notable for the absence of a grossly identifiable umbilical cord, with attachment to the placental vasculature by 2 vessels invested by amnion but without Wharton's jelly. We discuss the controversy regarding the requirement of an umbilical cord in the definition of acardiac twin and distinguish this case from placental teratoma.