Selection and characteristics of study population
311 pregnant women with RhD immunization were found eligible for inclusion in the OPZI 2.0 study. Figure 1 shows how the study population for the present analysis on HDFN severity in subsequent pregnancies was selected. In total, 62 women were included, with 155 pregnancies complicated by RhD antibodies and with a RhD-positive child (38 women with two, 19 women with three, four women with four and one woman with six pregnancies of RhD-positive children after her RhD antibodies were detected). Including two twins makes a total of 157 RhD-positive children. Table 1 shows the characteristics of included women and their children. To assess the risk of selection bias by selecting women with two or more subsequent pregnancies only, disease severity in the first immunized pregnancy of patients with and without a subsequent pregnancy was compared and showed a similar distribution (Table S1).