First-trimester ultrasonic diagnosis of twin reversed arterial perfusion sequence.

Twin reversed arterial perfusion (TRAP) syndrome sequence is a rare specific anomaly of twin gestation with fused placentae and umbilical anastomosis. This syndrome occurs once in very 35,000-48,000 births and has been described in the second trimester (23-29 weeks of gestation). We report on early sonographic diagnosis (10 and 12 weeks' gestation) of two cases of TRAP sequence, together with their umbilical cord Doppler studies.