Fetal hemolytic anemia and intrauterine death caused by anti-M immunization

Antibodies with anti-M specificity are detected in 10 percent of pregnant women with a positive antibody screen, but anti-M is only rarely associated with hemolytic anemia in the fetus.

This study reports on three pregnancies in one family that all resulted in severe fetal anemia. The first fetus died in utero with hydrops fetalis during the 20th gestational week and the second child was delivered after 28 weeks of gestation with hydrops fetalis and a hemoglobin level of 16 g per L whereas the third affected child was treated with intrauterine red cell (RBC) transfusions before delivery at 28 weeks of gestation.

The direct antiglobulin test was negative but anti-M in a low titer was detected through the three pregnancies, and its clinical relevance, which initially was uncertain, was confirmed by pronounced in vivo hemolysis in maternal blood of chromate ((51)Cr)-labeled M+ RBCs and normal survival of (51)Cr labeled M- RBCs.

It is concluded that anti-M immunization in a few cases may cause severe fetal hemolytic anemia and intrauterine death. It remains to be elucidated why a normally clinically insignificant antibody is this aggressive in a small proportion of cases. Because the condition is treatable, anti-M must be considered as a possible cause of fetal anemia and intrauterine death.