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Fetal and neonatal alloimmune thrombocytopenia (FNAIT)

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare blood disease that affects pregnancy and newborn babies. It occurs when a pregnant individual’s platelets do not match those of the fetus, causing an immune response that attacks the fetus’s or newborn’s platelets (a type of blood cell that helps with blood clotting). Most cases are mild, but in some severe cases FNAIT can cause bleeding in the fetus or newborn that can result in pregnancy loss or in a newborn’s death or disability. If a pregnancy is determined to be at risk for FNAIT, the pregnant individual may be treated with steroids and intravenous immunoglobulin, which is an antibody-rich product derived from donor blood that is administered through a vein. 


  • Low platelet count (thrombocytopenia) at birth
  • Bleeding
  • Bruising
  • Petechiae (small brown-purple spots on the skin caused by bleeding under the skin)
  • Intracranial hemorrhage (bleeding in or around the brain) 


FNAIT is caused by a mismatch in a type of protein on the surface of platelets between the pregnant individual and fetus. This platelet protein is called human platelet antigen (HPA). There are many types of HPA, and the most common is HPA-1a. If the fetus inherits an HPA type from the father that is not present in the mother, the fetus’s platelets are recognized as foreign by the mother’s immune system. The immune system responds by creating antibodies to attack and destroy the foreign platelets.

The antibodies produced by the pregnant individual pass through the placenta, where they enter the fetus’s blood and destroy the platelets.This causes a low platelet count (thrombocytopenia) and increases the risk of bleeding in the fetus and newborn. Bleeding episodes that affect the brain and major organs are serious complications and in some cases can result in pregnancy loss or death of the newborn.

How FNAIT is diagnosed 

There is no universal screening test for FNAIT, so first cases within a family are usually unexpected. However, recurrence of FNAIT in subsequent pregnancies is likely, and pregnant individuals with a sister whose child had FNAIT are also at risk. An assessment of risk in future pregnancies can be made by blood tests. 

A FNAIT diagnosis is made by platelet antigen typing, which involves testing the blood of each parent to determine if their platelet antigens are incompatible. The pregnant individual’s blood can also be tested for antibodies against the partner’s platelets. In some cases, the fetus’s platelet antigen type can be determined by testing the amniotic fluid or placenta tissue. If a newborn has a very low platelet count, doctors may use an ultrasound to determine if there is bleeding in or around the brain. This is called intracranial hemorrhage, and is the most serious complication of FNAIT, occurring in 10-15% of newborns with FNAIT.

Existing treatment

If a pregnancy is determined to be at risk of FNAIT, the pregnant individual may be treated with steroids and intravenous immunoglobulin, which is an antibody-rich product derived from donor blood that is administered to the patient through a vein. The individual may also be instructed against using nonsteroidal anti-inflammatory drugs (NSAIDs) and aspirin for the duration of the pregnancy. The fetus is closely monitored by ultrasound during pregnancy to watch out for brain bleeds. Although rare, brain bleeds in a fetus may require induction of early labor to adequately treat the infant.

The platelet counts of affected newborns are monitored closely for the first few days of life, and a platelet transfusion is performed if platelet counts drop below a specified threshold. Treatment may also include intravenous immunoglobulin and steroids.


FNAIT is estimated to occur in 1 in 1,000 to 10,000 live births worldwide.

Related conditions

FNAIT is the most common cause of severe thrombocytopenia in newborns. Thrombocytopenia is a condition characterized by a low blood platelet count, which can be caused by decreased platelet production, increased platelet destruction or platelets being trapped in the spleen. One of the most common causes of thrombocytopenia is immune thrombocytopenia (ITP), also known as idiopathic thrombocytopenic purpura. ITP is a condition in which a patient's platelets are targeted and destroyed by their own antibodies and can also be associated with complications in both the pregnant person and the fetus or newborn.

Community resources

The latest on our research

We are jumpstarting our FNAIT program to accelerate research on the condition and to ultimately improve treatment options. We welcome all patients and caregivers interested in participating and spreading the word to the FNAIT community.

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