ObjectiveTo determine if intrauterine intravascular fetal transfusion affects fetal umbilical venous endothelin levels.MethodsEndothelin concentrations were measured by radioimmunoassay in fetal umbilical venous blood obtained immediately before and after 36 fetal transfusions performed for Rh alloimmune hemolytic anemia. Umbilical venous pressures also were recorded before and after transfusion.ResultsThe mean (± standard deviation [SD]) gestational age at transfusion was 27.0 ± 4.6 weeks, whereas the initial and post-transfusion hematocrits were 23.3 ± 8.5% and 41.8 ± 6.3%, respectively. Post-transfusion endothelin levels correlated significantly with the volume of transfused blood (r= .41;P= .03) and with post-transfusion increases in umbilical vein pressure (r= .86;P< .001). Among fetuses undergoing initial transfusion, there were significant differences between mean (± SD) pre- and post-transfusion endothelin levels [3.6 (± 2.2) pg/mL versus 6.3 (± 4.0) pg/mL, respectively;P= .02]. In contrast, among fetuses undergoing a repeat fetal transfusion, no differences in mean (± SD) pre-versus post-transfusion endothelin levels were observed [3.8 (± 1.8) pg/mL versus 2.2 (± 1.77) pg/mL, respectively;P= .3)]. Step-wise multiple regression analysis identified order of transfusion as a significant predictor of change in endothelin levels from pre- to post-transfusion measurements (adjustedr2= .26;P= .003).ConclusionRapid expansion of fetal intravascular volume by intravenous transfusion of packed red blood cells with a high hematocrit enhances fetal endothelin levels in those fetuses undergoing initial but not subsequent transfusions.