Occurs in a diamniotic/monochorionic twin pregnancy, where blood is shunted through vein-to-vein and artery-artery anastomoses from the pump twin to the acardiac twin. Which term describes the acardiac twin?

• A. acardia twin
• B. pump twin
• C. donor twin
• D. recipient twin

Answer: (A)

In this situation, you’re looking at the twin reversed arterial perfusion (TRAP) sequence seen in monochorionic diamniotic pregnancies. One fetus—the pump twin—actively pumps blood, and abnormal vascular connections allow that blood to flow into the other fetus via artery-to-artery and vein-to-vein pathways. The recipient twin ends up without a functioning heart and is perfused by the pump twin’s circulation. Because this second fetus lacks a heart, it is described as acardiac. The term directly indicates the absence of a cardiac structure, which is the defining feature here.