OP15.02: Intertwin disparity in crown–rump length, gestational sac and yolk sac size and fetal heart rate at 7 weeks'gestation

To record intertwin disparity in biometric parameters and fetal heart rate at 7 weeks' gestation in mono- and dichorionic twin pregnancies. Twin gestations arising from ART programs were prospectively recruited at 7 weeks' gestation. Mean crown–rump length, gestational sac size, yolk sac size and fetal heart rate were recorded for each fetus and chorionicity was assigned. Ultrasonographic assessments were all carried out by two researchers using a GE Logic Expert 5 with a transvaginal ultrasound probe at a frequency of 8-MHz. Using a standardized protocol, three measurements were taken for each biometric parameter and the results averaged. Intertwin disparity in crown–rump length, gestational sac size and yolk sac size was expressed as a percentage of the larger co- twin's corresponding parameter. In the case of fetal heart rate, intertwin disparity was expressed as a percentage of the faster heart beat. This study population comprised 40 viable twin gestations prospectively recruited over a 10-month period. Chorionicity was determined as dichorionic in 37/40 (93%) of cases. Median intertwin disparity in crown–rump length was 16% (range 0–49%). Median disparity in gestational sac size was 10% (range 0–39%), in yolk sac size it was 8% (range 1–100%) and in fetal heart it rate was 5% (range 0–25%). Intertwin sonographic differences at 7 weeks' gestation are most notably reflected in a disparity in the crown–rump length of each fetus rather than gestational sac size, yolk sac size or fetal heart rates.