Familial occurrence of hemolytic disease of the newborn due to AO blood group incompatibility.

The extent of the familial occurrence of hemolytic disease of the newborn due to AO blood group incompatibility between mother and fetus was examined. The records of all families in which the mother had blood group O and two or more babies had blood group A, with one of them having been born at Women & Infants Hospital of Rhode Island in the index year of 1979, were examined. Eighty-nine of these families fulfilled criteria intended to exclude causes of hyperbilirubinemia other than AO blood group incompatibility. The peak bilirubin level in each infant was graded as normal, elevated, or severely elevated. The relations between birth order and peak bilirubin level, between peak bilirubin levels in the first and second babies in each family, and between gestational age and peak bilirubin level were assessed. The first two of these relations showed effects at the 0.10 and 0.05 significance levels, respectively. It is concluded that the risk of hyperbilirubinemia attributable to AO blood group incompatibility is greater for the second than for the first baby at risk in a sibship, that the risk is yet greater for the second if the first was also affected, and that, in the latter case, there is also increased risk of marked hyperbilirubinemia.