Aims: To investigate the incidence of severe fetal-to-maternal transfusion after delivery and to identify risk factors. Material and methods: In a prospective study at the Department of Obstetrics, Charité, Campus Virchow- Klinikum, Berlin, Germany, we analyzed the incidence of severe fetal-to-maternal transfusion (>10 ml) and fetalto- maternal hemorrhage (>25 ml) in Rh D-negative pregnant women after delivery of Rh D-positive infants. 942 women were included in the study and Kleihauer-Betke tests were performed. The results were compared to perinatal data. Results: Fetal-to-maternal hemorrhage occurred in 13 cases out of 942 (incidence of 1.3%) and severe fetalto- maternal transfusion in 61 cases (6.5%). In all of the cases with fetal-to-maternal hemorrhage, mothers were compatible with their infants in ABO-system. The incidence of fetal-to-maternal transfusion and its severe form was significantly higher in twin pregnancies (7/21 cases and 5/21 cases respectively, 33.3% and 23.8%) than in singleton pregnancies (22.5% and 5.9%, P<0.001). All other factors, such as maternal age, parity, ethnicity, mode of delivery, presentation, duration of first and second stage of labor, CTG, or Apgar score were not associated with an increased risk of severe fetal-to maternal transfusion. Conclusions: Twin pregnancy is the only independent risk factor for severe fetal-to-maternal transfusion. ABO-incompatibility between mother and infant seems to be protective against Rh D-alloimmunization.