The Benefits of Prenatal Diagnosis
Considering prenatal diagnosis once pregnancy is achieved is an important and complex decision. Although there are a wide variety of screening options available, prenatal diagnosis is the most accurate method for detecting chromosome abnormalities, such as Down syndrome. Diseases like cystic fibrosis, Tay-Sachs, sickle cell anemia, and thalassemias can be tested for if the parents are known to be carriers for these genetic diseases. Because prenatal diagnostic testing allows genetic experts to test placental cells directly, the results are diagnostic and specific for the fetus.
There are two different prenatal diagnostic tests, chorionic villus sampling (CVS) and amniocentesis. CVS is a procedure in which a small amount of tissue (chorionic villi) is obtained from the developing placenta at approximately 10-13 weeks of pregnancy. The tissue is then evaluated for chromosome abnormalities, and if indicated, specific genetic diseases. The primary advantage to CVS is that this test can be performed much earlier in pregnancy than amniocentesis. However, CVS does not detect neural tube defects (spina bifida, meningomyelocele or anencephaly). Therefore, patients who opt to pursue CVS undergo an AFP blood test and a high-resolution ultrasound later in pregnancy to screen for these defects. Also, approximately one percent of all CVS results will show a mixture of normal and abnormal chromosomes, which is called mosaicism. The majority of the fetuses in these pregnancies are normal, however additional testing, including amniocentesis, may be indicated.
CVS can be performed one of two ways depending on the location of the placenta within the uterus. The transcervical method is performed by inserting a thin catheter, guided by ultrasound, through the vagina and cervix to reach the chorionic villi. The transabdominal method is similar to amniocentesis. Using ultrasound, a thin needle is inserted through the mother's abdominal wall to obtain a small amount of tissue. In either case, this placental tissue is then sent for analysis.
Amniocentesis is typically performed between 16-20 weeks of pregnancy. Under ultrasound guidance, a thin needle is inserted through the mother's abdominal wall into the amniotic fluid surrounding the fetus. A small amount of fluid is then taken and analyzed for chromosome abnormalities, neural tube defects, and if indicated, specified genetic diseases. The main benefit to amniocentesis is that although it is performed later in pregnancy, it is possible to test for genetic disorders, including chromosome abnormalities and specific genetic diseases, AND neural tube defects, such as spina bifida, all at once.
Whether patients choose CVS or amniocentesis, it is possible to obtain the same information with either procedure. However for patients who choose CVS, it is necessary to do a follow up blood test and detailed ultrasound in the second trimester to rule out neural tube defects. It should be noted that the results from this blood test and ultrasound are not as conclusive on neural tube defects as the results from an amniocentesis. Because both procedures are considered invasive, meaning that it is necessary to enter the womb with either a needle or a catheter in order to obtain cells, there is a small risk of miscarriage due to the procedures. The risk for either CVS or amniocentesis is approximately 1/200. Diagnostic results from either procedure take about ten days to be completed.
Regardless of whether you are considering CVS or amniocentesis, genetic counseling is an important step in your overall decision-making process and in assessing your risk factors for genetic disorders. Genetic counselors are available to discuss in further detail the benefits, limitations, and risks for prenatal diagnostic testing in order for you to make the best decision for you and your family.
-- Kendall Glynn, MS, CGC, Certified Genetic Counselor, California Pacific Medical Center