How does an amniocentesis differ from chorionic villi sampling (CVS)?
Both amniocentesis and chorionic villi sampling are diagnostic prenatal tests and you only need to have one of these procedures during a pregnancy. While both tests give you information about the baby’s chromosomes, there are some important differences between the procedures.An amniocentesis differs from a chorionic villi sampling (CVS) in two ways:
1. Timing - An amniocentesis is done later in a pregnancy, during the second trimester. It is usually done between 15 to 20 weeks of pregnancy. A CVS test, on the other hand, is done in the first trimester, between 10 to 12 weeks of pregnancy.
2. Results obtained - Both amniocentesis and chorionic villi sampling will give you a picture of the fetal chromosomes which will diagnose Down syndrome with greater that 99% accuracy. However, with an amniocentesis, a substance called alpha fetoprotein (AFP) is also measured. The amount of AFP in the amniotic fluid can help determine if a fetus has an neural tub defects such as spina bifida or anencephaly.
Risk of Miscarriage
For many years, amniocentesis was thought to have a lower risk of miscarriage compared to CVS. However, it is now known that the risk of miscarriage due to these procedures is similarly low -– about 1/300 to 1/500, when performed by someone experienced with both procedures.
What are the advantages of amniocentesis?
The major advantages of an amniocentesis is that it has a lower risk of miscarriage and gives you more information. In addition to learning about the fetal chromosomes, the test also looks to see if the fetus has a neural tube defect.
What are the disadvantages?
The disadvantages of an amniocentesis is that the test is done in the later in pregnancy than a chorionic villi sampling. Also, there is a small but real chance of miscarriage. Some people feel that if they were to get abnormal results in the second trimester of pregnancy that it would be much harder to make a decision about continuing or not continuing a pregnancy. Others feel that if they would not act on the information or results that they would get from an amniocentesis, there is no reason to have a procedure that includes a small risk of miscarriage. It can sometimes be helpful to talk to a genetic counselor about your prenatal screening options.
Bottom Line
An amniocentesis test is the most common for prenatal diagnostic testing done today. It has the lowest risk of miscarriage and give you the most complete set of results. However, if the results are abnormal, there is not cure for Down syndrome or the other birth defects detected by this procedure. The only options at this point in pregnancy are to terminate or end the pregnancy, or continue to pregnancy knowing the baby has a diagnosis of Down syndrome. If your results are abnormal, you should talk with your doctor or with a genetics specialist about what your results mean and what your options are at this point in your pregnancy.
Sources:
Newberger, D., Down Syndrome: Prenatal Risk Assessment and Diagnosis. American Family Physician. 2001. American College of Obstetricians and Gynecologists (ACOG). Your Pregnancy and Birth, 4th Edition. ACOG, Washington, DC, 2005.
