Fetomaternal AB0 incompatibility and erythroblastosis fetalis.

Ninety-one of 330 fetuses undergoing intrauterine fetal transfusion (IUT) had sufficient residual fetal red cells in their cord blood samples that their AB0 blood groups could be determined. Ten of the 91 (11%) were AB0 incompatible with their mothers. The degree of Rh disease was severe in all 10. Three were hydropic in utero; 5 had the syndrome of hepatocellular damage; in 6, IUT were required between 22 and 26 weeks gestation. The expected incidence of AB0 incompatible pregnancies in which the fetuses required IUT if AB0 incompatibility had no effect on severity of Rh disease, is 9.7% versus the 11% observed. AB0 incompatibility has no effect on ameliorating severity of erythroblastosis after Rh immunization has developed. AB0 incompatibility between mother and father should not be taken into account when making decisions regarding the management of an Rh immunized pregnancy.