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The development of twin-twin transfusion syndrome complicates 5-35% of twin pregnancies with monochorionic placentation. Acardiac twinning is the most extreme manifestation of pathological vascular anastomoses between twins. An acardiac twin is a rare complication of multifetal pregnancy, reported in the literature with an incidence of 1% in monochorionic twin pregnancies, i.e. 1/35 000 pregnancies, and more than 400 cases have been described. We review the literature on this subject and report a special case of twin reversed arterial perfusion (TRAP) sequence in a twin pregnancy with the rare finding of a functional univentricular circulation pump in the acranius, its antenatal ultrasound diagnosis and postnatal findings. Remarkably, prenatally we could demonstrate two different arterial pulsations in the umbilical cord of the acranius. Etiological hypotheses of the TRAP sequence and new implications for risk-adapted therapeutic options are discussed. [ABSTRACT FROM AUTHOR]