Materno-fetal ABO incompatibility as a cause of spontaneous abortion

In a series of 288 spontaneous abortions occurring during the first 16 weeks of gestation, simultaneous karyotyping and ABO blood grouping of 555 of the parents were carried out. In 74 of the 288 chromosome-analyzed abortuses, the ABO blood group of the fetus was determined by the immuno-fluorescence technique and the mixed cell agglutinating reaction in fetal tissue. The results of the blood grouping were compared with the ABO blood group frequencies of 8818 blood donors from the same area. Among abortuses with normal karyotype, a significantly higher frequency of ABO incompatibility was found between mother and fetus (p smaller than 0.005) and also between mother and father (P smaller than 0.01) in comparison with abortuses with abnormal karyotype. Furthermore, the ABO blood group frequencies of the karyotypically normal fetuses deviated significantly from those of fetuses with abnormal karyotypes (P smaller than 0.001). No significant difference was found when the total ABO frequencies of the abortuses and of their parents were compared with the frequencies in the control group. It is concluded that the ABO incompatibility between mother and fetus is likely to be a cause of early spontaneous abortions, but almost exclusively in chromosomally normal abortuses. In the present series of cases, the maximum fraction of abortions caused by materno-fetal ABO incompatibility is estimated to be 18%.