The assessment of fetal blood samples.

Fetal blood sampling under ultrasound control is rapidly expanding the study of human fetal biology. Pure fetal blood is required for prenatal diagnosis, establishment of reference ranges for biologic measurements, and assessment of fetal welfare. We present here the methods that we have developed to detect contamination in more than 1500 samples. These include hematologic indexes, blood smear, erythrocyte antigens, beta-human chorionic gonadotropin, and coagulation factor assays. The tests are relatively simple, inexpensive, and widely available. No single test is reliable in all situations, and it is necessary to perform all the tests on each sample of fetal blood. In the clinical setting where irrevocable action may be taken as a consequence of our results, we require absolute assurance of the purity of the sample.

Assessment of fetal blood volume for computer-assisted management of in utero transfusion.

We performed 41 intravascular ultrasound-guided fetal transfusions in a total of 20 pregnancies with erythroblastosis fetalis or alloimmune thrombocytopenia. On the basis of this experience, we developed a computer-assisted procedure for determining the volume to be transfused, which provides an adequate final concentration. Fetal weight was estimated using ultrasound measurements. Fetoplacental blood volume was estimated from the regression line: fetoplacental volume (ml) = 1.046 + fetal weight (g) X 0.14. The volume to be transfused was calculated using the simple dilution formula: Vtransfused = Vfetoplacental.(Cfinal - Cinitial)/Ctransfused where C is the hematocrit or platelet count. The entire procedure is computerized, simple and rapid, and avoids resorting to repeated intermediate sampling. The dilution formula used appears to be more reliable than a formula taking into account the volume added, even in erythroblastosis fetalis where relatively large volumes are injected. This suggests rapid plasma loss during the procedure.