The results of repeated Rh antibody titrations on 65 sensitized Rh-negative pregnant woman carrying Rh-positive fetuses are presented and analyzed. Considering that a rise in antibody titer antenatally could result only from a leakage of fetal blood into the maternal circulation, the findings indicate that such a leakage occurs antenatally in about 1 out of every 3 or 4 normal pregnancies. Similarly, evidence is presented indicating that fetal blood leaks into the maternal circulation in about 1 out of 3 deliveries. Since, previously, it has been shown that approximately 40 per cent of Rh-negative individuals can be sensitized by two injections of Rh-positive blood spaced four months apart, one can calculate the chance of sensitization resulting from pregnancy with an Rh-positive fetus. The calculations show that in Rh-negative women who have received a previous injection of Rh-positive blood, there is about one chance in 12 or 13 that the first baby will be erythroblastotic (cf. Levine and Waller).16 In women who have never received any injection of blood which might sensitize them, the chance that the first baby will be erythroblastotic is negligible, because for this to occur it would be necessary for blood to leak into the maternal circulation antenatally on at least two occasions spaced four months apart, once to prime the patient and the second time to stimulate the appearance of antibodies.8 This never occurred in the present series of cases. It was also calculated that in Rh-negative primiparae who have never received an injection of Rh-positive blood, the chance of Rh antibodies appearing after the birth of an Rh-positive baby is approximately 2 out of 45, and, as a matter of fact, this has been found to occur by other workers10 in 2 out of 65 cases. It was further calculated that in Rh-negative women who had never received an injection of Rh-positive blood, the chance of their second baby being erythroblastotic is about 1 in 20, which agrees well with the actual observations. By extending the calculations, one could also estimate the chance of an erythroblastotic baby at the third, fourth, and later pregnancies.

Five cases are described in detail in order to illustrate the value of periodic antenatal Rh antibody titrations in the management of sensitized Rh-negative women.

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