Fetal and neonatal alloimmune thrombocytopenia (FNAIT)

A healthy neonate is born after an uneventful pregnancy. The cord platelet count is 12 × 10⁹ per L. Twelve hours later he has a seizure, and the cranial ultrasound shows a large parenchymal haemorrhage. The diagnosis that was missed antenatally — because there is no routine screening programme — is FNAIT, and the registrar who knows this disease protects the next pregnancy. FNAIT rewards the candidate who knows the antibody, the feared complication, the antenatal treatment, and the delivery plan.[1][8]

Overview and definition

FNAIT is the platelet alloimmune disease of the fetus and newborn. Maternal sensitisation to a paternally inherited fetal human platelet antigen (HPA) produces an IgG alloantibody that crosses the placenta and is cleared by the fetal reticuloendothelial system along with the antibody-coated fetal platelets. The result is isolated fetal and neonatal thrombocytopenia — and, in a minority, devastating intracranial haemorrhage.[1][7]

Two features make FNAIT unlike haemolytic disease of the fetus and newborn, and examiners test them:[7][8]

• The first affected pregnancy is usually the index presentation. There is no routine antenatal HPA screening programme in current practice, so the disease is typically recognised only when a neonate presents with severe thrombocytopenia, bleeding, or — worst case — an intracranial haemorrhage.
• The severity rises with each successive antigen-positive pregnancy. Antibody titres and disease severity typically climb, so the next pregnancy is the opportunity for prevention — and the one the obstetric team can influence.[1][6]