Diagnosis and Management of Fetal and Neonatal Alloimmune Thrombocytopenia: An Update 2025.

Background: Antibodies of the mother, which are directed against paternal antigens on platelets of the child, can lead to the destruction of the fetal blood cells in the circulation after diaplacental passage. The clinical picture of fetal-neonatal alloimmune thrombocytopenia (FNAIT) is characterized by bleeding, of which intracranial bleeding is particularly feared. Our understanding of the pathophysiology of FNAIT and its targeted prophylaxis and therapy has improved significantly in recent years. Summary: FNAIT by anti-HPA-1a is the best studied. How exactly the mother is immunized is not known for certain, but, in clinically apparent cases, immunization usually occurs in the first pregnancy of an HLA-DRB3*01:01-positive, HPA-1a-negative woman. There is no convincing basis for assigning immunization against HPA-5b and against HLA class I any significance in the development of fetal thrombocytopenia. Newborns of mothers with anti-HPA-1a present a broad clinical picture ranging from isolated, clinically unremarkable thrombocytopenia to intracranial hemorrhage (ICH; in approx. 1-10% of cases). ICH usually occurs intrauterine (before week 28). There are indications that, in addition to the fetal platelets, the placenta can also be affected by anti-HPA-1a. As there are no screening programmes, the index diagnosis of FNAIT is random. It is made by serological and genetic laboratory tests. Predicting outcome in a subsequent pregnancy is problematic if the child is antigen-positive. Key Messages: With a first-born child with severe thrombocytopenia, the probability of a recurrence of severe thrombocytopenia is around 70%. Without ICH, the probability of ICH in the subsequent pregnancy is low but not zero, and with ICH the recurrence rate is high. There is no established laboratory diagnostic method to predict the severity of thrombocytopenia or the occurrence of ICH. Prophylaxis with immunoglobulins (IVIgs) is considered effective. Pharmaceutics that block placental transport are currently undergoing clinical trials and may replace IVIgs in the future. Intrauterine platelet transfusions should no longer be performed. For the mature, thrombocytopenic newborn without internal hemorrhage, a platelet transfusion is advisable for platelet counts <25 g/L.