Blood | Fetal and neonatal alloimmune thrombocytopenia

Fetal thrombocytopenia refers to an insufficient number of platelets in the fetal blood circulation. Just like in adults, platelets in the fetus are responsible for forming blood clots to prevent ongoing bleeding after injury or trauma. If there are insufficient circulating platelets, the fetus could develop large bleeds in the brain and this can lead to a stroke while the fetus is still inside the womb.

Essential Information

  • Platelets are small, disk-shaped fragments (or pieces) of cells that are found in large numbers in the fetal and adult circulations. They play an important role in forming blood clots to prevent ongoing bleeding after an injury.
  • Fetal thrombocytopenia refers to a lack of platelets in the fetal blood circulation. Severe thrombocytopenia can make it difficult for the fetal blood to clot, which might lead to serious bleeding in the fetal brain.
  • Fetal thrombocytopenia may result from a condition called fetal and neonatal alloimmune thrombocytopenia (FNAIT), where the mother’s immune system forms antibodies that attack and destroy the fetal platelets.
  • When severe thrombocytopenia is diagnosed, weekly immunoglobulin infusions to the mother may be considered.


Thrombocytopenia is a lack of platelets in the fetal circulation. Platelets are responsible for blood clotting to prevent ongoing bleeding after an injury.

In a condition called fetal and neonatal alloimmune thrombocytopenia (FNAIT), the mother and fetal platelets are incompatible. The mother’s immune system therefore recognizes the fetal platelets as foreign and mounts an immune attack by forming antibodies to these cells. These antibodies can cross through the placenta and reach the fetal circulation. In this way, they can destroy the fetal platelets and cause the fetus to become thrombocytopenic.

Fetal thrombocytopenia cannot be diagnosed on ultrasound, but may present with ultrasound findings of a bleed around the fetal brain (called an intracranial hemorrhage). In these cases, the fetal outcomes are very poor. The mother will not have any symptoms.

Sometimes, fetal thrombocytopenia is suspected because of the outcomes of a previous pregnancy, if the mother has already delivered a child affected with thrombocytopenia, FNAIT or a bleed in the brain. Blood taken from the mother and the father may provide important information about the maternal-fetal platelet incompatibility.

Mild and moderate thrombocytopenia may not affect the fetal wellbeing, and the long-term outcome of these infants can be excellent.

In severe thrombocytopenia, however, fetal blood clotting is significantly impaired and minor trauma may lead to fetal bleeds in the brain (intracranial hemorrhage). These bleeds can lead to a stroke while the fetus is still in the womb, and can result in fetal death or significant long-term developmental delay.



Fetuses at high risk for thrombocytopenia due to FNAIT should be cared for in coordination with a Maternal Fetal Medicine specialist. Preventative treatment with weekly immunoglobulin infusions to the mother, starting in the second trimester of pregnancy, may be indicated. Fetal blood sampling may be considered in the third trimester to document fetal platelet levels and to adjust further treatment. This strategy has significantly reduced the need for fetal intrauterine platelet transfusions. The risk for severe thrombocytopenia causing intracranial hemorrhage after such preventative therapy is less than 5 per cent and is associated with reassuring long-term outcomes.


Referral Information (for physicians)

inclusion criteria:

  • Confirmed maternal platelet antibodies with or without a history of a previously-affected pregnancy.
  • Incompatible platelet phenotype in partner.


IVIG administration can often be administered locally and on an outpatient basis, in shared care with Mount Sinai Hospital. Fetal blood sampling at 28 and 36 weeks may be recommended. Local patients will have shared care between the referring centre and Mount Sinai Hospital. Delivery will be planned at Mount Sinai Hospital.