Fetal / Neonatal alloimmune thrombocytopenia (FNAIT) is a rare syndrome caused by maternal IgG antibody directed against a fetal platelet antigen inherited from the father. Approximately 1 in 1000 pregnancies is affected, with about half of the cases occurring in first pregnancies. Although HPA-1a is the dominant human platelet alloantigen (HPA) incompatibility causing FNAIT, a significant number of cases are caused by other HPA incompatibilities. Investigation includes HPA antibody testing and HPA genotyping. Platelet crossmatching is also performed between maternal sample and paternal platelets when available.