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 from 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. Fetal MCA Doppler waveforms were assessed biweekly from 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 can 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.