Amniocentesis is an accurate but invasive prenatal procedure to diagnose genetic, chromosomal or immune-related disorders in the fetus.
Initially used as a standard tool in the diagnosis of fetal and neonatal alloimmune thrombocytopenia (FNAIT) during pregnancy, the use of amniocentesis has reduced significantly in recent years, replaced by non-invasive medical options of less risk to the fetus.
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 amniocentesis?
Amniocentesis is an invasive medical procedure performed on the fetus during pregnancy to diagnose a variety of conditions, including genetic disorders, congenital abnormalities and infections. It can also be performed to reduce the volume of amniotic fluid in rare cases where there is an excess amount.
A long needle is inserted into the amniotic sac with the guidance of ultrasound imaging and extracts amniotic fluid rich in fetal cells, transudates, fetal urine and lung secretions. The timing of the test is important, with the safest window considered between 15 and 20 weeks’ gestation.
How is amniocentesis used in FNAIT?
Amniocentesis was traditionally used to diagnose FNAIT by performing fetal genotyping on the discarded fetal cells found in amniotic fluid. However, in recent years, the focus has shifted to safer, less invasive alternatives such as cell-free fetal DNA testing (cffDNA). In pregnancies suspected of FNAIT, amniocentesis is therefore rarely used.
Risks of amniocentesis in FNAIT
As an invasive procedure, amniocentesis can lead to complications that cause discomfort for the pregnant mother, fetal distress and potentially fetal death. The most common complications include leaking amniotic fluid, bleeding, severe cramps, infection, preterm labor and miscarriage.
In FNAIT, amniocentesis increases the risk of the intervention, upping the risk of fetal loss and complications by 11%.
Alternatives to amniocentesis
Fetal genotyping by cffDNA, testing for maternal antibodies and maternal and paternal HPA genotyping to assess the risk of FNAIT are all used in diagnosing FNAIT in pregnancy.
If FNAIT is suspected, the current standard of care is to administer IVIG with or without steroids as soon as possible to delay the onset of severe thrombocytopenia. Following delivery, the newborn will receive platelet transfusions to boost blood platelet levels and, in most cases, symptoms will be resolved.
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