Among the retrospective studies that included only HDFN patients (n = 7), three evaluated HDFN in general (including ABO alloimmunization cases) [17, 21, 34], two evaluated severe HDFN [26, 35], and another two evaluated HDFN due to Rh alloimmunization [29, 30]. Here, ABO is linked to fetal erythroblastosis.