[Twin Reversed Arterial Perfusion Sequence (TRAP) : a circulatory paradox causing complications in monochorial twin pregnancies].

The TRAP sequence («Twin Reversed Arterial Perfusion Sequence») is a rare condition found in approximately 1 to 3 % of monochorionic twin pregnancies. It results from the existence of placental vascular anastomoses (arterio-arterial and veno-venous) between a normal twin and an acardiac twin (or with a dysfunctional heart). The normal twin then serves as a «pump» for the abnormal twin who is retrogradely perfused. Throughout pregnancy, the well-being and survival of the pump twin is compromised by different mechanisms (risk of high-output heart failure, premature delivery, premature rupture of membranes, chronic hypoxia and delayed intrauterine growth). The diagnosis can be made from the 11th week of gestation using ultrasound and Doppler examination of the umbilical arteries. Although it is established that the mortality of pump twins is around 50 % in the absence of treatment, there is currently no consensus on the treatment methods nor on the timing to be respected. Some advocate an expectant attitude with close monitoring. Others prefer an intervention to interrupt blood flow to the acardiac twin. Intrafetal therapies by laser or radiofrequency are the most used methods at present. They appear safe and effective.