A critical evaluation and discussion of certain technical aspects of the acid elution method and the immunofluorescent technic for the demonstration of fetal erythrocytes in maternal blood is presented. With appropriate precautions a close agreement between the two methods was obtained. Using one or both methods as applicable, base line data concerning transplacental passage were obtained in 622 unselected women, compatible with their offspring in the ABO system and not sensitized against the Rh factor. The intermittent entry of fetal erythrocytes into the maternal blood in small quantities was found to be a physiologic event. Postpartum, fetal erythrocytes were demonstrated in 50 per cent of the mothers. In approximately 10 per cent of the series, large fetal losses estimated to range from 0.5 to 40 ml. were observed. Massive transplacental hemorrhage was detected in nearly 1 per cent of the cases by examination of the maternal blood and usually but not always by overt anemia in the fetus. It was found to be the cause of stillbirth in one case. The evidence suggested that fetal bleeds usually began well before the onset of labor.

Though labor itself under pathologic circumstances may be associated with transplacental hemorrhage, under ordinary conditions it was found to be of little or no significance with regard to the entry of fetal erythrocytes into the maternal circulation.

Survival of fetal erythrocytes in the maternal circulation after delivery was generally found to correspond to the expected life span of red corpuscles. Apparent shortening of survival observed in several instances was probably due to the occurrence of the original spill some time before delivery. In two instances persistence of fetal erythrocytes well beyond the normal life span was observed. Both cases involved massive transplacental hemorrhage and fetal anemia with marked erythroblastemia, and mother and child were compatible to an exceptional degree with respect to blood group factors. The findings suggested the successful transplant of replicable erythroid precursor cells from fetus to mother with prolonged maternal tolerance, an interpretation supported by the results of skin homografts from offspring to mother in one case. The implications of these findings are discussed.

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