BACKGROUND: Fetomaternal alloimmune thrombocytopenia (FMAIT) is the commonest cause of severe thrombocytopenia in term neonates but its management remains controversial.

STUDY DESIGN AND METHODS: A 7‐year prospective observational study of 200 cases of FMAIT evaluated the relationship between human platelet antigen (HPA) antibody specificity, clinical presentation, morbidity, mortality, and therapeutic interventions in the antenatal and postnatal period, with long‐term follow‐up of neonates with intracranial hemorrhage (ICH).

RESULTS: In 1148 referrals for FMAIT, HPA antibodies were confirmed in 200 (17%). The commonest specificities were anti‐HPA‐1a, 150 (75%); anti‐HPA‐5b, 31 (15.5%); and anti‐HPA‐15b, 8 (4%). Of 123 (62%) cases (two sets of twins) with no previous history of FMAIT, intrauterine deaths occurred in 5: anti‐HPA‐1a alone, 3; in combination with anti‐HPA‐5b, 1; and anti‐HPA‐15b, 1. Of the 120 live neonates, 103 had severe thrombocytopenia and 17 (14%) developed ICH (anti‐HPA‐1a, 13; anti‐HPA‐5b, 3; anti‐HPA‐15b, 1). Postnatal care varied widely with 37 percent of neonates receiving random rather than HPA‐1a and ‐5b–negative platelets. Of the remaining 77 cases with a history of FMAIT, 40 received intrauterine transfusions. Six (15%) of these fetuses died in utero and an additional 2 developed ICH postnatally. Of the 19 children with ICH, 1 (anti‐HPA‐15b) died on Day +1, and neurologic sequelae persist in 13 (mean follow‐up, 2.5 years).

CONCLUSION: HPA‐1a antibodies are most commonly implicated in severe thrombocytopenia but HPA‐5b and HPA‐15b antibodies can also result in poor outcome. Postnatal transfusion management is extremely variable, and fetal transfusions are associated with significant morbidity and mortality.