Alloimmunization in twin pregnancies.

Of 118 twins born of 59 alloimmunized mothers from 1944 to 1984, 65 were Rh-positive and six died. One twin in one of the 59 sets and four sets of twins referred from elsewhere underwent intrauterine fetal transfusion. The Manitoba twin and two pairs of the four referred sets survived. Fetal transfusions were begun at 22 2/7 and 24 1/7 weeks' gestation in one surviving set and 23 3/7 weeks in both twins of the other surviving set. Three twins of the four who died were hydropic at referral. There would have been a 62.5% chance of survival if they had been referred at 23 weeks' gestation. ABO incompatibility in one twin of three sets had no effect on the amelioration of severity of Rh disease. In three sets, gross hemoglobin disparities were due to twin-to-twin transfusion. Differences in degree of severity of disease in the other four dichorionic sets may have been due to differences in sex, Rh constitution, amount of fetal erythropoiesis, and fetal hepatocellular function.