Fetal and neonatal alloimmune thrombocytopenia (FNAIT) affects approximately 1 in 1000 pregnancies, and it can be fatal for a fetus or newborn. FNAIT occurs as a result of an incompatibility between the mother and fetus and affects the blood platelets.
What is FNAIT?
Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare but serious condition that affects 0.1% of pregnancies in which a pregnant mother’s immune system produces antibodies against the platelets of her fetus. This occurs when a fetus inherits platelet antigens from the father that are not compatible with the mother, typically involving a protein called human platelet antigen (HPA). The mother’s immune system recognizes the fetal platelets as foreign, attacking and destroying them, leading to low platelet levels (thrombocytopenia) in the fetus or newborn.
What is the role of platelet antigens in FNAIT?
Blood platelets are small colorless cell fragments that stop bleeding by forming clots. A low platelet count means that blood doesn’t clot properly, leading to uncontrolled bleeding and hemorrhage.
There are 35 identified human platelet antigens (HPAs). They are found in the platelet membrane and can activate the development of alloantibodies when exposed to foreign platelets with different HPAs. The most common platelet antigens in FNAIT are HPA-1a antibodies, followed by HPA-5b.
Learn more about FNAIT causes and risk factors
What are maternal antibodies?
When the pregnant mother’s immune system detects the presence of foreign platelet antigens HPA inherited from the father in her fetus, this activates the development of maternal antibodies. These anti-HPA antibodies cross the placenta and attack the fetus’s blood platelets, progressively breaking them down and causing thrombocytopenia (a very low blood platelet count). This puts the fetus at risk of bleeding and hemorrhage.
The maternal antibodies stay in the mother’s system and may therefore affect future pregnancies.
Testing for platelet antigens and antibodies
There is no standard prenatal screening program for FNAIT. However, if bleeding occurs during pregnancy and FNAIT is suspected, non-invasive diagnostic testing is preferred. If the fetus has thrombocytopenia, invasive testing can lead to bleeding and even miscarriage. A recent non-invasive way of testing for FNAIT during pregnancy is next generation sequencing. It can determine the fetal HPA-1a early in pregnancy. Another non-invasive approach is identifying the fetal HPA-1a by testing cell-free fetal DNA in maternal plasma.
HPA antibody and genotype testing following delivery when symptoms present in newborns involves screening and identification of the maternal anti-HPA antibody and HPA genotyping of mother, father and newborn.
It is important to confirm diagnosis because FNAIT can occur in subsequent pregnancies. Pregnancies at risk of FNAIT are considered high-risk as the pregnant mother already has the antibodies and they can cross the placenta as early as 16 weeks’ gestation. Early diagnosis allows for proactive treatment, which can prevent severe complications.
