Severe fetal and/or neonatal thrombocytopenia caused by an antibody from the mother attacking the baby’s platelets in utero. This disease is usually abbreviated as “NAIT,” and has also been called “neonatal alloimmune thrombocytopenic purpura (NATP).” NAIT occurs when a mother lacks a specific platelet antigen (HPA-1a or “PlA1” in 85% or more of cases) and is exposed to that antigen through either pregnancy or transfusion. Mom forms an IgG antibody against the target antigen. In a future pregnancy (or sometimes in the same pregnancy that induced the antibody), the antibody is transported across the placenta and binds to the baby’s antigen-positive platelets. The affected fetus or neonate has severe thrombocytopenia (commonly less than 10,000/mcL), and 20% have intracranial hemorrhage. Several treatments have been described, including intravenous immunoglobulin in mom (prenatal) and baby (antenatal). If the baby must be transfused, there are two main options: 1) Antigen-negative platelets from a random donor, usually obtained from a blood center, or 2) Mom’s platelets (which are antigen-negative), washed to remove the offending antibody, and irradiated to prevent Transfusion-associated Graft-vs-Host Disease.