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Prenatal Diagnostic Testing Procedures
A genetic counseling appointment is scheduled for any person considering a diagnostic prenatal procedure, such as chorionic villus sampling or amniocentesis, in order to review the benefits, risks and limitations of the procedure. Available genetic testing options are also reviewed during this session. The different possible procedures are:
Many genetic tests and other testing options exist for individuals undergoing a prenatal procedure. Genetic counselors review these options before your procedure is performed. They will help you understand the benefits and limitations of these tests and help you decide which are right for you and your family.
Chorionic Villus Sampling
Chorionic villus sampling (CVS) is an ultrasound-guided procedure that samples tissue (chorionic villi) in the placenta outside the amniotic sac. CVS is typically performed in the first trimester between 11 and 13 weeks gestation by a trained specialist and can provide earlier results than those provided by amniocentesis which is typically performed in the second trimester starting at 16 weeks gestation.
The procedure can be performed via the abdominal wall (transabdominally) or via the cervix (transcervically), depending on the location of the placenta. There is a small risk of bleeding, infection and miscarriage. Patients should understand the risks of the procedure and should discuss the potential risks and benefits with their health care provider and a genetic counselor.
CVS allows for diagnostic genetic testing to be done on placental cells, which are typically genetically identical to the fetal cells. The type of testing done after a CVS varies in each case, but it can provide information on fetal karyotype, to detect Down syndrome, and certain genetic syndromes such as sickle cell disease or Tay Sachs disease.
Genetic amniocentesis is an ultrasound guided procedure that samples a small amount of amniotic fluid (20-30 mL; 4-6 teaspoons) from the sac around the baby. An amniocentesis is typically performed starting at 16 weeks gestation by a trained specialist. An amniocentesis allows diagnostic testing to be done on fetal cells, compared to the CVS where diagnostic testing is done on the placental cells. Diagnostic tests can be done on the sample to identify fetal chromosomal abnormalities (such as Down syndrome), prenatally diagnosable genetic diseases, and most neural tube defects (spina bifida).
There is a small risk of bleeding, infection and miscarriage. Patients should understand the risks of the procedure and should discuss the potential risks and benefits with their health care provider and a genetic counselor.
Fetal Blood Sampling
Percutaneous umbilical cord blood sampling (PUBS) allows doctors to sample fetal blood under ultrasound guidance. The test is usually performed after 18 weeks gestation. PUBS provides a means for rapid and accurate genetic testing, analysis of blood disorders (e.g. anemia, hemophilia, alloimmune thrombocytopenia, identification of congenital infection), or early detection of metabolic diseases that might possibly not be detected by amniocentesis or chorionic villus sampling in certain cases. This procedure is also used to perform blood transfusion (e.g. Rh incompatibility) and to directly administer medication to the fetus.
The procedure itself is similar to an amniocentesis procedure and involves placement of a small needle into the uterus for a fetal blood sample from the umbilical cord. PUBS has been reported to have a pregnancy loss rate of approximately 1 percent to 2 percent. Potential risks include spontaneous miscarriage, infection, hemorrhage, preterm labor, and premature delivery.
Chromosomes are the structures in our cells that contain our genetic information. Routine chromosome analysis involves looking at the chromosomes of a baby by analyzing cells from the amniotic fluid (obtained from amniocentesis), placenta (obtained from CVS) or other fetal tissue. This analysis evaluates for missing or extra genetic information, which can lead to genetic syndromes, such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome), caused by an extra copy of either chromosome 21, 18 or 13, respectively. The chance for having a baby with a chromosomal trisomy increases as women get older, but it can occur at any age. For example, at age 28 a woman has approximately a 1 in 1,000 risk for Down syndrome compared to a 1 in 100 risk at age 40.
Chromosomal microarray analysis
Chromosomal microarray analysis (CMA) a comprehensive prenatal genetic testing option and is performed in conjunction with chromosome analysis. CMA has the ability to detect small missing or extra missing pieces of chromosome material (microdeletions and microduplications) that are associated with more than 180 genetic conditions, including most of the conditions that can be found on a routine chromosome analysis. The risk for most of these conditions does not depend on parental age. If one combines the risk for having a baby with any of these conditions, it is often higher than the risk for having a baby with Down syndrome, especially when the ultrasound shows that the fetus has one or more birth defects or is not growing well.
Fluorescence in situ hybridization
Fluorescence in situ hybridization (FISH) is a more rapid way to test for common chromosome problems such as Down syndrome, trisomy 18, trisomy 13 and Turner syndrome. Confirmatory chromosome analysis or CMA is always performed in addition to FISH testing.
Neural tube defects
Spina bifida is an abnormality of the neural tube and occurs in 1 in 1,000 babies. Many neural tube defects, such as spina bifida, can be evaluated during pregnancy by analyzing amniotic fluid via an amniocentesis. Ultrasound and blood work can also be helpful tools in detecting spina bifida during pregnancy. Neural tube defects cannot be evaluated with other prenatal procedures (i.e. CVS).
Other specific testing
Testing for a specific genetic condition may be indicated depending on family history or pregnancy history and may include testing for a single gene, a panel of genes or whole exome/genome sequencing (WES/WGS). WES/WGS is the broadest test for diagnosing single gene disorders.
A genetic counselor will meet with you prior to your procedure to determine whether additional testing is indicated for your pregnancy.