Learn The BasicsFetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare disease that may put the life of an infant at risk. It is caused by the immune system of a pregnant woman mistakenly attacking her baby’s platelets, which are blood fragments that help form blood clots and stop bleeding. The disease puts the baby at risk of heavy bleeding and/or bruising before or just after birth.
What causes FNAIT?
Platelets have several proteins on their surface, which are inherited from the parents and are different for each person.
If a baby inherits a platelet surface protein from their father that is not present in the mother, the mother’s body may recognize that protein as foreign and start making antibodies against it. These antibodies may cross the placenta during pregnancy and enter the circulation of the fetus attacking its platelets.
Since platelets are essential to prevent or stop bleeding, this may lead to an increased risk of bleeding for the infant.
What is the risk of FNAIT?
The risk of FNAIT is calculated to be around one in 1,500 pregnancies. Bleeding in the brain of the fetus due to FNAIT, the most severe complication of the condition, is thought to occur in one in 10,000 pregnancies.
In 50% of FNAIT cases, it occurs in the first pregnancy, and there is a 90% risk of a mother developing FNAIT after experiencing it in a previous pregnancy.
FNAIT risk factors
Risk factors associated that could lead to the development of FNAIT include:
- The entry of the baby’s blood into the circulation of the mother during delivery
- A previous miscarriage involving leakage of the platelets of the fetus into circulation of the mother
- Prior exposure of the mother to adult platelets, such as during a blood transfusion
- The release of cell particles from the placenta
A fetus is also at high risk of developing FNAIT if they have an older sibling with a platelet count below 20×109/L. If the older sibling experienced bleeding in the brain due to FNAIT before birth, the fetus has an increased risk of also experiencing a brain bleed before birth.
Certain genetic makeups can also increase the risk of FNAIT. For example, more than 90% of women who develop antibodies against their baby’s HPA-1a have the so-called DRB3*01:01 allele of the HLA gene. Also, the presence of two HLA alleles (DRB3*01:01 and DRB4*01:01) of the HLA gene may increase the risk and severity of FNAIT.
Finally, there may be a link between the blood type of the mother and the severity of thrombocytopenia or low platelet count in the newborn baby with FNAIT. In fact, research has shown that women who have antibodies in their blood against their baby’s HPA and the blood group O seem to be at lower risk of having a baby with severe FNAIT than women with blood group A.
Which populations are most at risk?
There are several platelet proteins that can be targeted by the immune system of the mother, and these often vary by race and ethnic origin. In White people, the most common protein is called human platelet antigen (HPA)-1a, while in African Americans it is HPA-2 and HPA-5 that commonly cause FNAIT. In Japanese people, HPA-4 and HPA-5 are mostly associated with FNAIT.
FNAIT prevention
A pregnant woman who has previously had a baby with FNAIT will likely be referred to a clinic that specializes in fetal-maternal medicine, and the pregnancy should be monitored closely.
She may be treated with intravenous immunoglobulins with or without steroids from 12 to 16 weeks of gestation to dampen the immune system’s response and reduce the risk of the new baby also being affected by FNAIT.
