The mother is homozygous for HPA-1b. Approximately 98% of the population are positive for the HPA-1a antigen, and only 2% are HPA-1b/1b homozygous. The maternal antibodies cross the placenta and cause variable degrees of fetal or neonatal thrombocytopenia. In contrast to Rh-isoimmunization, in which the first pregnancy is not typically affected, FNAIT can affect the first pregnancy. In patients without a history of FNAIT, the diagnosis is not suspected because the mother is asymptomatic, unless ultrasound shows evidence of a fetal hemorrhage.

Intracranial hemorrhage can occur at platelet levels of 10,000-20,000 microL. If a prior pregnancy is affected, the recurrence risk is about 80%, with subsequent pregnancies. However, subsequent pregnancies are not necessarily more severely affected, except if the previous pregnancy has been associated with intracranial hemorrhage. In those patients, the risk of recurrence is higher and at an earlier gestational age. The overall risk of intracranial hemorrhage is approximately 10-20%, with 75% of cases occurring antenatally.

Alloimmune thrombocytopenia is usually suspected if a prior child was born with a very low platelet count (thrombocytopenia), bruises, bleeding from various sites, or an intracranial hemorrhage. Sophisticated laboratory testing using blood from both the mother and the father must be done in specialized laboratories to confirm the diagnosis.

The antenatal management of patients at risk for FNAIT includes determining the platelet antigen incompatibility between the mother and the father of the baby. Fetal platelet genotyping may be performed via chorionic villus sampling or amniocentesis. A non-invasive cell-free DNA test to assess fetal platelet genotype is likely to replace invasive testing. If the fetus is at risk, special medical protocols including weekly intravenous immunoglobulin (IVIG) and steroids have been successful in raising the neonatal platelet count compared to prior children from the same mother.