Middle cerebral artery peak systolic velocity in monochorionic and dichorionic twin pregnancies.

OBJECTIVE: The purpose of this study was to compare middle cerebral artery (MCA) peak systolic velocity (PSV) values in monochorionic (MC) and dichorionic (DC) twin pregnancies. METHODS: This was a prospective cohort study in which MCA Doppler evaluation was performed in unselected twin pregnancies at time of routine sonography between 28 and 32 weeks. Pregnancies with known fetal anomalies, twin-twin transfusion syndrome, and red cell alloimmunization or other conditions associated with anemia were excluded. The intertwin MCA PSV difference, defined as the larger minus smaller PSV value within a pair, was compared in MC and DC pregnancies and was correlated with estimated fetal weight and birth weight discordance. Statistical analyses included Spearman correlation, analysis of variance, a t test, and a chi(2) test. RESULTS: Doppler indices were analyzed from 48 twin pregnancies, of which 32 (67%) were DC and 16 (33%) were MC. There was no difference in proportion of values above or below the singleton median for either the larger or smaller DC or MC twins (all P > or = .3). The median intertwin MCA PSV difference was 4.9 cm/s in MC pregnancies and 4.5 cm/s in DC pregnancies (P = .6). There was no significant correlation between the MCA PSV difference and either estimated fetal weight discordance or birth weight discordance in either MC or DC pregnancies (all P > or = .3). CONCLUSIONS: Middle cerebral artery PSV values in uncomplicated twin pregnancies are comparable with published singleton norms, with a median intertwin MCA PSV difference of approximately 5 cm/s. We found no significant correlation between the intertwin MCA PSV difference and discordance in MC or DC twin gestations.