Chorionic Villus Sampling (CVS)
This procedure is generally performed between 10 and 13 weeks of pregnancy. Because the fetus and the placenta develop from the same cells, they are usually genetically identical. Testing the placenta, therefore, provides information about the fetus. Using ultrasound guidance, a small amount of placental tissue is withdrawn and used for fetal chromosome analysis. Depending on the location of the placenta, the procedure is performed either through the cervix or through the mother's abdomen.
In approximately 1% of patients, the chromosome studies of the placental cells may not be entirely representative of the fetal chromosomes (a condition called placental mosaicism). If this is suspected, amniocentesis is usually offered to confirm these results.
Amniocentesis
Amniocentesis is performed after 14 weeks of pregnancy. Using ultrasound guidance, a needle is inserted into the bag of fluid around the fetus, and a small amount of fluid is withdrawn. Fetal cells that are floating in this fluid can be isolated and used for fetal chromosome analysis.
What are the risk of these tests?
The risk of miscarriage due to CVS is approximately 1/150. When CVS is performed at 10 weeks of gestation or later, there is no procedure-related risk of birth defects. The risk of miscarriage due to amniocentesis is approximately 1/200-1/300; when performed at 14 weeks or later, there is no procedure-related risk of birth defects. Rarely, fetal cells obtained from amniotic fluid may not grow enough for chromosome studies to be performed. If this occurs, another amniocentesis will be offered.