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The page below is a sample from the LabCE course Advances in Noninvasive Prenatal Testing For Down Syndrome and other Trisomies. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Diagnostic Tests: CVS and Amniocentesis

Chorionic villus sampling (CVS):
The placenta is an organ connecting the developing fetus to the mother’s uterine wall to allow for oxygen and nutrient exchange and to eliminate waste via the mothers blood supply. It attaches to the wall of the uterus and the fetus’s umbilical cord develops from the placenta.
As previously discussed, CVS is an invasive diagnostic prenatal test in which a sample of chorionic villi is removed from the placenta for testing. The sample can be taken through the cervix (transcervical) or the abdominal wall (transabdominal). The chorionic villi are wispy projections of placental tissue that share the baby's genetic makeup.
The major benefit of CVS is that it can be performed early in the pregnancy usually between weeks 10 and 13 of gestation (earlier than other prenatal diagnostic tests, such as amniocentesis). The tissue samples obtained via the CVS technique are then prepared and analyzed for chromosomal abnormalities, such as is present in Down syndrome, CVS can also be used to test for many other disorders including cystic fibrosis.
Chorionic villus cells obtained from the CVS technique are collected and cultured from chorionic villi. Testing of the samples is typically performed using cytological karyotype analysis involving cytogenetic staining of the chromosomes in the specimen with subsequent microscopic analysis. More details on test methods for CVS analysis are presented in the next section.
Amniocentesis:
Amniocentesis is an invasive prenatal diagnostic procedure for obtaining a sample of amniotic fluid from the uterus. It uses ultrasound to locate the fetus position. Amniotic fluid surrounds and protects the fetus during pregnancy. It contains fetal cells and other chemicals produced by the fetus.
Amniocentesis involves placing a needle directly into the uterine cavity and aspirating some amniotic fluid containing fetal cells (amniocytes) which are cultured, harvested, and subjected to karyotype and/or genetic analysis. The amniotic fluid extracted typically contains cells from the fetal skin, lungs, and urinary tract. The procedure is usually performed between week 15 and 20 of pregnancy. Amniocentesis done before week 15 of pregnancy has been associated with a higher rate of complications. The procedure is usually reserved for those women considered at higher risk of carrying a fetus with a chromosomal or genetic abnormality.
Specimens obtained from amniocentesis are tested using karyotype chromosomal analysis as well as other select genetic biochemical and molecular analysis. More details on the test methods are presented in the next section.
Advantages, disadvantages, and risks of CVS and amniocentesis:
The major advantage of CVS is that it can be performed early in the pregnancy, usually between weeks 10 and 13 of gestation (earlier than other prenatal diagnostic tests, such as amniocentesis). A disadvantage of CVS is related to the source of cells being analyzed. The cells primarily arise from the placenta and not the fetus itself, which in about 1-2% of cases can lead to discrepancies in the test results. This discrepancy is caused by a condition called confined placental mosaicism (CPM) which can lead to cytogenetically ambiguous test results and may require follow-up testing using amniocentesis. CPM is discussed in the next section.
The main disadvantage of amniocentesis is that it typically is not performed until after the 15th week of gestation. In addition, when compared to CVS, amniocentesis normally retrieves fewer viable cells, thereby precipitating long culture times and consequently a longer turnaround time for test results. One of the advantages of amniocentesis over CVS is that it can identify neural tube defects involving the spinal column or brain.
Both CVS and amniocentesis carry similar risks including the following:
  • Miscarriage: Risk is estimated to be about 1% for CVS and 0.6% for amniocentesis.
  • Rh sensitization: If mother has Rh negative blood, then preventative Rh immune globulin injections may be needed.
  • Infection: Very rarely a uterine infection may occur.