Neonatal platelet count (×10⁹/L) in relation to the mother’s dose of the HLA-DRB3*01:01 allele. Platelet counts of children born of HLA-DRB3*01:01–negative women, who carried the HLA-DRB3*02:02 allele, are marked with filled circles. Women who were heterozygous or hemizygous (1 dose) gave birth to newborns with lower platelet counts than children born of HPA-1a–immunized HLA-DRB3*01:01–negative women (0 dose), but the neonatal platelet count was higher than in children born of women who were HLA-DRB3*01:01 homozygous (2 doses). The horizontal bars denote the median values in each group of women. Values below the dotted lines, equivalent to platelet counts of 50 and 30 × 10⁹/L, respectively, indicate particularly high risk of ICH. Of the children born of the 13 HLA-DRB3*01:01 homozygous women, 8 and 6 had platelet counts below these limits. For the 106 women with 1 dose of HLA-DRB3*01:01, these figures were 28 and 38, respectively. All the children born of women who lacked HLA-DRB3*01:01 had platelet counts >50 × 10⁹/L. Kruskal-Wallis test was used to calculate P_global. The other P values were calculated by using the Mann-Whitney U test.