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Notes: Objective To determine the proportional reduction per day in the number of fetal and donor red blood cells from the fetal circulation after intrauterine intravascular transfusions.Design A retrospective study of 302 transfusions in 101 fetuses.Setting The Department of Obstetrics and Gynaecology of the University Medical Centre Leiden, The Netherlands.Methods We measured the haematocrit in fetal samples both before and after repeated intravascular intrauterine transfusion in fetuses with alloimmune disease. The percentage of fetal erythrocytes was determined in Kleihauer-Betke stained smears. The decline of fetal, donor and mixed red blood cells was calculated by dividing the proportional decrease of the haematocrit values by the number of days between transfusions, also after correction for changes in fetoplacental volumes. Results (given as mean [SD]) are derived from the proportional changes of haematocrit per day.Results The interval between the first and second transfusion (15.5 days [SD 5.2]) was shorter than between subsequent transfusions (means ranging from 21.4 to 21.9 days; P〈inlineGraphic alt="leqslant R: less-than-or-eq, slant" extraInfo="nonStandardEntity" href="urn:x-wiley:14700328:BJO818:les" location="les.gif"/〉 0.0001). The decline per day of mixed, and of donor red blood cells, calculated without corrections for volume changes did not differ from those corrected for volume changes resulting from the transfusion and from fetal growth (correction factor 1.1 [SD 0.4]). Since the coefficient of variance is smaller for the uncorrected decline values, this type of calculation is preferable for clinical purposes. The disappearance of fetal erythrocytes after the first transfusion (6.1 %/day [SD 2.9]) was faster than that of mixed fetal and donor red blood cells (3.2%/day [SD 1.2]; P 〈 0.0001) and of donor cells alone (1-4%/day [SD 1.6]; P 〈 0.0001). The decline of the mixed red blood cell population became the same as that of the donor cells (2.2%/day [SD 0.8]) after the second transfusion. This decline of donor cells was higher than after the first transfusion (1.4%/day [SD 1.6]; P 〈 0.05). After the first transfusion the fetal erythrocytes disappeared faster after transplacental puncture of the umbilical cord (6.6%/day [SD 2.8]) than after transamniotic punctures (5.4%/day [SD 2.7]; P= 0.05). The mixed red blood cell also decreased faster (3.5%/day [SD 1.3] versus 2.8%/day [SD 0.9]; P 〈 0.01).Conclusion The fast disappearance of fetal erythrocytes, especially after transplacental punctures, shows that the interval between the first and second transfusion needs to be shorter than that for intervals between subsequent transfusions. The number of donor erythrocytes declines by approximately 2% per day.