To assess whether maternal HPA 1a alloimmunization is associated with birthweight. Design. A retrospective observational cohort study. Setting. The national reference laboratory for clinical platelet immunology at a university hospital. Population. 165 HPA 1a incompatible pregnancies identified from a recent screening study of 100 448 women (124 pregnancies) and the national reference laboratory for clinical platelet immunology (41 pregnancies). Methods. A linear mixed model analysis was used to assess whether maternal anti-HPA 1a antibodies were associated with birthweight. A generalized linear model was used to assess maternal anti-HPA 1a antibodies as risk factor for small-for-gestational age neonates. Both models were adjusted for gestational age at time of delivery, maternal age, parity, smoking habits during pregnancy, preeclampsia, diabetes mellitus and fetal sex. Main outcome measures. Maternal anti-HPA 1a antibody as risk factor of reduced birthweight and small-for-gestational age neonates. Results. The level of maternal anti-HPA 1a antibodies was significantly associated with birthweight and risk of small-for-gestational age neonates after correcting for confounding variables (p<0.001). However, this association was only significant for boys. When the mother had high levels of anti-HPA 1a antibodies during pregnancy, the adjusted mean birthweight in boys was 530g lower compared with anti-HPA 1a antibody negative pregnancies (p<0.001). Conclusions. A linear relation between maternal anti-HPA 1a antibody levels and reduced birthweight in boys was demonstrated. Reduced birthweight should be considered a possible complication of fetal and neonatal alloimmune thrombocytopenia.