Twin Reversed Arterial Perfusion (TRAP) Sequence Phenotypes: A Comprehensive Visual Analysis

Twin reversed arterial perfusion (TRAP) sequence is a rare and severe complication of monochorionic twins, occurring in approximately 1 in 35 000 pregnancies. It is characterized by the presence of a severely abnormal, nonviable acardiac twin and a structurally normal pump twin, which sustains the circulation of both fetuses through abnormal vascular placental anastomoses. This pictorial essay aims to illustrate the sonographic and Doppler findings associated with TRAP sequence, emphasizing the diagnostic prenatal features. Typical findings include the absence of cardiac activity in the acardiac twin, structural anomalies such as acardia, acrania, underdeveloped upper body, and paradoxical reversed flow patterns in the umbilical artery. Early diagnosis and close ultrasound monitoring are crucial for selecting antenatal treatment, including expectant management and fetal intervention. By presenting illustrative cases, this article provides a comprehensive overview to bolster the understanding of the different associated phenotypes and recognition of TRAP sequence amongst clinicians and sonographers.