Learn The BasicsFetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare disease that is not routinely tested for during pregnancy.
The disease is usually diagnosed when a baby is born with low platelet counts. Signs of low platelets include bleeding, bruising or pinpoint round spots that form on the skin also known as petechiae. If this occurs in the first pregnancy, there is a higher risk of the baby being affected in a second pregnancy, which should be closely monitored.
Blood tests for the baby
If a newborn baby is suspected of having FNAIT, a blood test can be performed to test for thrombocytopenia or low platelet counts.
FNAIT is caused by a pregnant woman’s immune system mistakenly attacking proteins that are found on the surface of her fetus’ platelets (cellular fragments that are crucial for clotting). This happens when the fetus inherits platelet proteins from the father that are different from the mother’s, leading to an immune response against these incompatible proteins.
This immune attack causes the fetus’ platelets to be destroyed causing thrombocytopenia, putting the fetus at high risk of heavy bleeding. The risk of bleeding may continue for a few weeks after birth until the mother’s antibodies are cleared from the baby’s blood.
There may also be other reasons why a baby has thrombocytopenia and additional tests are needed to confirm an FNAIT diagnosis.
Blood tests for the mother
In case of FNAIT suspicion, the mother’s blood can be tested for antibodies against the platelet proteins of her baby.
In White people, the most common antibody causing FNAIT is called anti-HPA-1a, which targets a platelet protein called human platelet antigen (HPA)-1a.
There are also other antibodies that can cause FNAIT such as anti-HPA-2, anti-HPA-3, anti-HPA-5, and anti-HPA-15. These may be more commonly associated with the risk of FNAIT in different human populations.
The fact that a woman has these antibodies in her blood does not necessarily mean that her baby will have FNAIT and the levels of antibodies do not always correlate with the severity of thrombocytopenia.
The blood sample from the mother can also be used for genetic tests to identify which proteins she has on her platelets and see whether these are compatible with the proteins on the surface of her fetus’ platelets.
Blood tests for the father
Due to the increased risk of bleeding, blood is not usually drawn from a fetus to identify its genotype.
Instead, a blood test is conducted on the father of the fetus to work out which genes they may inherit and which protein they may express on the surface of their platelets.
For example, if the father is heterozygous for the gene coding for HPA1, i.e. has two different versions of the gene, there is a 50% chance that the fetus’ platelets will be compatible with those of the mother.
Imaging tests
If a baby is suspected of having FNAIT at birth and blood tests have revealed that they have very low platelet counts, the doctor may order an ultrasound to check for bleeding in the brain, also called intracranial hemorrhage or ICH, the most feared complication of FNAIT.
