True risk of fetal/neonatal alloimmune thrombocytopenia in subsequent pregnancies: a prospective observational follow-up study.

OBJECTIVES To assess neonatal platelet counts by comparing alloimmunised pregnancies from a Norwegian screening and intervention study with subsequent pregnancies from the same women. DESIGN Prospective observational follow-up study. SETTING A university hospital. POPULATION HPA-1a immunised women from a large Norwegian screening study that gave birth to one or more children after the screening study ended (2004-2012). METHODS Follow-up of maternal anti-HPA-1a antibody levels and neonatal platelet counts from the screening pregnancies were compared with subsequent pregnancies. None of the women received antenatal intravenous immunoglobulin (IVIG) treatment and neonatal platelet counts were therefore comparable. MAIN OUTCOME MEASURES Change in neonatal platelet counts from one HPA-1a incompatible pregnancy to the next. Maternal anti-HPA-a1 antibody levels from one HPA-1a incompatible pregnancy to the next. RESULTS Forty-five incompatible subsequent pregnancies were identified. Overall, the neonatal platelet count in the subsequent pregnancy was improved (18%), unchanged (52%), or worse (30%), compared with the corresponding screening pregnancy. There was one case of fetal intracranial haemorrhage (ICH) identified in the screening (intrauterine fetal death detected at 30 weeks of gestation) and no ICH cases recorded for the subsequent pregnancies. In cases where the platelet count was lower in the subsequent pregnancy, the maternal anti-HPA-1a antibody level was higher compared with the screening pregnancy. In comparison, the maternal antibody level was lower in subsequent pregnancies where the platelet count improved. CONCLUSIONS In contrast to what is often stated, we found that the neonatal platelet count was increased or unchanged in the majority of subsequent pregnancies of HPA-1a-immunised women.