Fetal blood sampling is an invasive procedure used to diagnose fetal and neonatal alloimmune thrombocytopenia (FNAIT). It is performed for fetal genotyping and to measure fetal blood platelet levels in blood collected from the umbilical vein.
If platelet levels are very low, the fetus is at risk of developing potentially life-threatening complications, such as intracranial hemorrhage (ICH).
However, the use of fetal blood sampling in pregnancies suspected of FNAIT has declined in recent years as lower-risk, non-invasive diagnostic methods have become increasingly more accurate.
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 fetal blood sampling?
Fetal blood sampling or cordocentesis is an invasive medical procedure where a small amount of blood is extracted from the umbilical vein. It involves the insertion of a long, thin needle into the pregnant mother’s abdomen.
For accuracy, the needle is guided by ultrasound through the abdomen, into the uterus and then into the umbilical vein. A small sample of blood is taken and then tested for genetic mutations, blood diseases, infections or other disorders. In the case of FNAIT it is a diagnostic test that also guides treatment choices by measuring the platelet count of the fetus.
What are the risks of fetal blood sampling?
While fetal blood sampling is valued for its accuracy, it has a high level of associated risk for the fetus.
Platelets are blood cells that form blood clots and promote healing but when the platelet count is low–as in FNAIT–the blood doesn’t clot. The slightest injury can therefore lead to uncontrolled bleeding and hemorrhaging in the fetus, with the risk of long-term consequences or death.
Fetal blood sampling when thrombocytopenia is present can put the fetus at risk of bleeding from the puncture site, fetal bradycardia, tamponade of the cord and bleeding complications in the fetus. These complications can result in fetal distress, leading to early labor, miscarriage and in some cases, fetal death.
Alternatives to fetal blood sampling
In first FNAIT-affected pregnancies, it is important to confirm which platelet antigen is concerned. Noninvasive fetal genotyping for HPA-1a, the most common cause of FNAIT, can be conducted by cell-free fetal DNA in maternal plasma. It is an accurate test that has very little risk. In the case of other HPAs, maternal and paternal genotyping can predict the fetal genotype.
If a diagnosis of FNAIT is confirmed, intravenous immunoglobulin (IVIG) and corticosteroids are administered to suppress the maternal antibodies that are destroying the fetus’s platelets. This delays or prevents the onset of complications of thrombocytopenia including intracranial hemorrhage (ICH), which potentially has the most severe consequences. Regular fetal monitoring by ultrasound is also key during a FNAIT pregnancy to detect possible bleeding or hemorrhage.
