Neonatal alloimmune thrombocytopenia (NAIT) occurs in fetuses and neonates. Results from the transplacental passage of maternal alloantibodies directed against fetal platelet antigens. Although uncommon, NAIT is the leading cause of severe thrombocytopenia in the newborn.

NAIT results when fetal platelets contain an antigen inherited from the father that the mother lacks, usually human platelet antigen (HPA)-1a incompatibility. The mother develops antigens against the paternal antigen, and the antibodies cross the placenta and bind with fetal platelets.

The mother is usually asymptomatic. The clinical range of disease is dependent on onset and severity of the thrombocytopenia in the fetus, ranging from mild asymptomatic thrombocytopenia to severe thrombocytopenia. Most cases are mild, and typical presentation consists of a neonate with widespread petechiae or ecchymosis. The bleeding into the skin is typically apparent a few hours after birth, or in some cases, after circumcision or hematoma formation at injection sites. In severe cases, neonates may present with bleeding into the lungs or gastrointestinal system. Intracranial hemorrhage (ICH) may lead to death in severe cases.

Typically not suspected until after delivery of the first child of a couple, unless there is a family history of NAIT. NAIT has a high recurrence rate and typically presents with more severe manifestations in following pregnancies. Fetal ICH can be detected during ultrasound examination or prenatal screening.
 
The most rapid and effective treatment in neonates with severe hemorrhage and/or thrombocytopenia is platelet transfusion to help maintain platelet counts. Antenatal management typically consists of administration of intravenous immunoglobulin / corticosteroid or intrauterine platelet transfusions. Both of these treatment methods may help prevent severe thrombocytopenia and complications.