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While recognizing the aforementioned risks, the American College of Obstetricians and Gynecologists recommend that prenatal screening, or diagnostic testing for aneuploidy via procedures like amniocentesis or chorionic villus sampling, be discussed with and offered to all patients regardless of maternal age or risk profile. [5]
It entails sampling of the chorionic villus (placental tissue) and testing it for chromosomal abnormalities, usually with FISH or PCR. CVS usually takes place at 10–12 weeks' gestation, earlier than amniocentesis or percutaneous umbilical cord blood sampling. It is the preferred technique before 15 weeks. [2]
Amniotic fluid is removed from the mother by an amniocentesis procedure, where a long needle is inserted through the abdomen into the amniotic sac, using ultrasound guidance such that the fetus is not harmed. Amniocentesis is a low risk procedure, with risk of pregnancy loss between 1 in 1,500 – 1 in 700 procedures.
To reduce the risk of intraamniotic infection, antibiotics are supplied through the intravenous access about 30–60 minutes before the procedure. If movement of the fetus is a risk to the success of the procedure, the fetus may be paralyzed using a fetal paralytic drug. [10] This image shows anterior blood sampling from the umbilical cord.
The demand for cordocentesis tests is diminishing because it has been replaced with CVS and Amniocentesis, which carry less risk. The procedure is typically done via needle into the mother's abdomen, in combination with ultrasound for guidance, to obtain a blood sample from the umbilical cord of the fetus. [18]
When screening is positive, chorionic villus sampling (CVS) or amniocentesis testing is required to confirm the presence of a genetic abnormality. However this procedure carries a small risk of miscarriage so prior screening with low false positive rates are needed to minimize the chance of miscarrying.
There are 3 possible ways to test the fetal antigen status. Cell-free DNA, Amniocentesis, and Chorionic Villus Sampling (CVS). Of the three, CVS is no longer used due to risk of worsening the maternal antibody response. Once antigen status has been determined, assessment may be done with MCA scans. [citation needed]
These procedures are highly accurate because they are taking a sample directly from the fetus; however, there is a small risk for the woman to miscarry and lose the pregnancy as a result. Both CVS and amniocentesis require the pregnant woman to visit a genetic specialist known as a maternal-fetal medicine specialist who will perform the procedure.