A Rare Case of Acardiac Acephalus Fetus in Monochorionic Twins

Twin reversed arterial perfusion (TRAP) sequence is a rare obstetric complication of monochorionic twin pregnancies characterized by an acardiac fetus hemodynamically dependent on a structurally normal "pump" twin. It is frequently misdiagnosed as intrauterine fetal demise due to absent cardiac activity in the anomalous twin. A 28-year-old pregnant woman (G2P1L1) with a history of a previous cesarean section presented at 24 weeks of gestation following an external ultrasound suggesting single fetal demise in a monochorionic twin pregnancy. Institutional re-evaluation with color Doppler ultrasonography demonstrated retrograde arterial flow into an anomalous fetus with rudimentary structures, confirming the acardius acephalus variant of TRAP sequence. The pregnancy was managed conservatively with bi-weekly fetal surveillance and Doppler monitoring. At 35 weeks, the patient developed intrahepatic cholestasis of pregnancy. An elective cesarean section at 36 weeks resulted in a healthy live male neonate weighing 1.7 kg and a 1.33 kg acardiac mass. Postnatal echocardiography of the pump twin was normal, and both mother and neonate had an uneventful postoperative recovery. The acardiac-to-pump twin weight ratio in this case was 0.78, exceeding the classic 70% threshold associated with severe pump twin cardiac failure, polyhydramnios, and preterm birth. Despite this high volume load, the pump twin remained hemodynamically stable in utero, highlighting that individual placental vascular architecture significantly influences actual clinical outcomes. Accurate chorionicity assessment and early Doppler evaluation are essential to prevent misdiagnosis. Close antenatal surveillance facilitates successful expectant management and favorable outcomes in selected high-risk TRAP sequence cases.