Prenatal diagnostic tests include chorionic villus sampling (CVS) and amniocentesis. They can provide definitive information about the baby’s chromosomes and/or other genetic conditions. These tests have some risks, the most significant of which is miscarriage.
During a genetic counseling visit, you will receive information about a variety of prenatal testing options. Some of these options do not have major risks to mother or baby. A genetic counselor will not require you to have any testing you do not want. Rather, a genetic counselor will provide you with complete information about testing options so you can make decisions that are right for you and your family.
Chorionic Villus Sampling (CVS)
Chorionic villus sampling (CVS) is a test that can be done early in a woman’s pregnancy to determine if the fetus has certain conditions. CVS can tell whether a baby has Down syndrome, cystic fibrosis or other genetic or chromosomal conditions. DNA from a CVS test can also be used as part of a paternity test. CVS can usually be performed between 10 and 13 weeks gestation — much earlier than other tests.
During CVS, a small sample of tissue is removed from the placenta for testing. The procedure can be performed in two ways — transabdominally (through the abdomen) or transcervically (through the cervix) — depending on where the placenta is located. Both methods of CVS are performed with ultrasound guidance.
Good candidates for CVS include women who:
- Will be 35 years of age or older at the time of delivery
- Had an abnormal first trimester screening result
- Have a family history of a genetic condition such as cystic fibrosis or sickle cell anemia
- Have had a prior birth involving a genetic condition
- Had an abnormal finding on an ultrasound
While most CVS tests are performed through the abdomen, about 5 percent to 10 percent need to be performed transcervically (through the cervix) because of the placenta’s location. It takes much more training and experience to conduct a CVS test transcervically; our maternal fetal medicine specialists have undergone this extensive training. (A woman whose doctor is not trained to perform the procedure transcervically must wait many more weeks until amniocentesis can be performed to answer questions about her baby’s health.)
A study conducted in 1992 by the National Institutes of Health. determined that both methods of CVS testing are equally safe. Still, CVS does carry risks, including a risk of miscarriage. Therefore, women considering a CVS test should carefully weigh the risks and benefits.
Amniocentesis Diagnostic Procedure
Amniocentesis is a diagnostic procedure that involves withdrawing a small amount of fluid from the sac surrounding the fetus. A long, thin needle is inserted through the abdomen and into the uterus, and a small amount of fluid is taken from the sac. Before the procedure, the exact location of the fetus is determined by ultrasound.
Amniocentesis can be used to diagnose a large number of genetic and chromosomal abnormalities in the fetus. It is also helpful for diagnosing the severity of Rh incompatibility, lung maturity, and neural tube defects (such as spina bifida).