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Objective: To measure the effects of acute large increases of the hematocrit on fetal peak arterial and maximum venous blood flow velocities., Methods: Middle cerebral artery peak flow velocities and umbilical vein maximum flow velocities were measured before, immediately after, and 12-24 h after intrauterine transfusions. All measurements were standardized for gestational age., Results: Complete measurements were obtained at 60 intrauterine transfusions. The mean hematocrit before intrauterine transfusion was 0.19 l/l and after 0.40 l/l. The middle cerebral artery peak flow velocity decreased immediately after transfusion in 59 of the 60 cases. There was a rise in umbilical vein maximum flow velocity immediately after intrauterine transfusion in 37 of the 60 cases. The sensitivity of middle cerebral artery peak flow velocity for severe anemia before intrauterine transfusion was 54% and the specificity 57%. The sensitivity of umbilical vein maximum flow velocity for severe anemia before intrauterine transfusion was 67% and the specificity 57%., Conclusions: An acute large increase of the fetal hematocrit significantly decreases middle cerebral artery peak flow velocity. The effect on umbilical vein maximum velocity is, however, unpredictable., (Copyright 2003 S. Karger AG, Basel)