Key Points Free fetal DNA can accurately be used to determine the fetal red cell genotype. A false positive elevation of the middle cerebral artery peak systolic velocity can occur in the early second trimester in a red cell alloimmunized pregnancy. In most cases, fetal intrauterine transfusions should be limited to pregnancies with a documented positive fetal genotype for the involved antigen.
Moise et al. (2026) studied this question.