Twin reversed arterial perfusion sequence with Holoacardius acephalus and pump twin demise at 17 weeks of gestation

A 26-year-old primigravida presented at 17 weeks of gestation with absent fetal cardiac activity on ultrasonography in one of two previously identified monochorionic diamniotic twins. Evacuation specimen confirmed the classic triad of Twin Reversed Arterial Perfusion (TRAP) sequence. Panel A shows the pump twin, a pale, macerated fetus with preserved craniofacial and limb morphology, consistent with intrauterine fetal demise secondary to high-output cardiac failure. Panel B depicts the acardiac twin showing the holoacardius acephalus subtype - complete absence of the head and upper thorax, with rudimentary lower limb appendages, resulting from chronic retrograde perfusion of deoxygenated blood through arterio-arterial placental anastomoses. A single, shared monochorionic placenta with visible chorionic vascular anastomoses is also visible in the image. TRAP sequence arises exclusively in monochorionic fetal gestations via arterio-arterial anastomoses that redirect deoxygenated flow from the pump twin to the acardiac co-twin in a retrograde fashion. The caudal structures are preferentially perfused, resulting in cranial and cardiac agenesis. Untreated, pump twin mortality approaches 50-75%. Radiofrequency ablation procedure or bipolar cord coagulation before 16 weeks significantly improves survival. This case highlights the critical importance of early surveillance in monochorionic twin gestation and first-trimester Doppler assessment.