['Severe fetomaternal transplacental hemorrhage increases the risk of fetal anemia. In the third trimester, the syncytiotrophoblast becomes thinner, especially in areas where it comes into intimate contact with villous capillaries, and forms a vasculosyncytial membrane. Our aim was to determine whether ABO compatibility puts the fetus at a greater risk of severe fetomaternal hemorrhage.', 'Case study.', "A tertiary care center. Sample and methods. Between 2003 and 2007, we evaluated eight cases of severe fetomaternal transfusion. The Kleihauer-Betke test was used for diagnosis of fetomaternal hemorrhage. We evaluated blood group compatibility between the mother and fetus and assessed the perinatal outcome. The Fischer's factorial test was used for testing a hypothesis.", 'The incidence of adverse outcomes following transplacental hemorrhage was 75% (six of eight). There were two perinatal deaths and four infants were affected by post-hypoxic damage of varying severity. Fetomaternal ABO compatibility was present in seven of the eight cases. The risk of severe fetomaternal hemorrhage was significantly increased when there was ABO compatibility between the mother and fetus. This was associated with a very poor perinatal outcome.'] Does aBO fetomaternal compatibility pose a risk for massive fetomaternal transplacental hemorrhage?