Reference ranges for middle cerebral artery peak systolic velocity in monochorionic diamniotic twins: a longitudinal study

Objectives The role of middle cerebral artery (MCA) peak systolic velocity (PSV) has become established in the management of fetal anemia. To date, singleton reference ranges have also been used in twin pregnancies. However, in monochorionic twin pregnancies, normal ranges for cerebral blood flow may differ front those in singletons owing to intertwin blood exchange. We aimed to establish gestational age-specific reference ranges for MCA-PSV in monochorionic diamniotic (MCDA) twin pregnancies, to compare them with previously reported singleton reference ranges, and to establish terms for calculating conditional reference intervals appropriate for individual serial measurements. Methods In a prospective longitudinal study we examined 50 uncomplicated MCDA pregnancies that were recruited between 11 and 14 weeks of gestation. Petal MCA Doppler waveforms were assessed biweekly front 15 weeks of gestation onwards. Multilevel modeling was used to estimate gestational age-specific reference ranges for MCA-PSV and terms for conditional reference intervals were established. Results Based on 824 observations in 100 fetuses, normative ranges from 15 to 37 weeks of gestation were constructed. Median MCA-PSV increased with advancing gestational age. Measurements between 18 and 37 weeks were comparable to those in singletons. However, before 18 weeks MCA-PSV values were higher in MCDA twin pregnancies compared with singleton references. Conclusions Between 18 and 37 weeks of gestation, reference ranges of singletons calf be used to assess fetal anemia in MCDA twin pregnancies. Prior to 18 weeks the application of singleton references may lead to an increased number of false-positive diagnoses of presumed fetal anemia in MCDA twin pregnancies. Copyright (C) 2009 ISUOG. Published by John Wiley & Sons, Ltd.