What is prenatal diagnosis?
Prenatal diagnosis is a way for your doctor to determine, before birth, whether your fetus has a problem such as Down syndrome. Amniocentesis and chorionic villus sampling (CVS) are tests that help find genetic disorders before birth. Some parents are at increased risk of having a baby with a genetic disorder or other problem and may want to have one of these tests. Knowing about problems before the baby is born may help parents make decisions about health care for their infant. Certain problems can be treated before the baby is born, while other problems may need special treatment right after delivery. In some cases, parents may decide not to continue the pregnancy.
Do all pregnant women have these tests?
No. Amniocentesis or CVS may be considered in situations in which the parents are at increased risk of having an infant with a genetic disorder. The tests may be useful if you are 35 years of age or older by the time your baby is due. Women older than 35 years of age have an increased risk of having a baby with a chromosome abnormality, such as Down syndrome. The tests may also be useful if you have previously had a child with Down syndrome or another disorder, such as spina bifida. The test may also be helpful if you or your partner are known carriers of a genetic disorder, such as cystic fibrosis.
How is amniocentesis performed?
During amniocentesis, a sample of amniotic fluid (the fluid around the baby) is removed from your uterus and sent to a laboratory for evaluation. Amniocentesis is performed by inserting a thin needle through your abdomen into your uterus (womb) and withdrawing a small amount of fluid. Your body will make more fluid to replace the fluid that is taken out. The baby will not be hurt during the procedure. Some women feel mild cramping during or after the procedure. Your doctor may tell you to rest on the day of the test, but usually you can resume normal activity the next day.
How is CVS performed?
CVS is performed by removing a small sample of the placenta (nourishment for the baby) from the uterus. It is removed with either a catheter (a thin tube) or a needle. Local anesthesia is used for this test to reduce pain and discomfort. The sample of placenta may be obtained through the cervix. A catheter is inserted into the vagina and through the cervix and the sample is withdrawn. The sample can also be obtained by inserting a needle into the abdomen and withdrawing some of the placenta. Most women feel fine after the test, although some may have mild bleeding (spotting) afterward.
When are the tests performed?
Amniocentesis is usually performed during the 15th week of pregnancy or later. CVS is usually performed between the 10th and 12th weeks of pregnancy.
Is one test better than the other?
There are some situations in which amniocentesis is more appropriate than CVS. Amniocentesis is preferred if you have previously had a baby with a neural tube defect or if you or your partner have a neural tube defect. (CVS doesn't detect neural tube defects.)
Amniocentesis may be better if the results of other tests (such as the alpha-fetoprotein test) during your pregnancy have been abnormal. The blood screening test may show that your infant is at greater risk of having a neural tube defect or a disorder such as Down syndrome. CVS may be better if you and your doctor want to know the test results during your first trimester.
Are there risks involved with these tests?
Amniocentesis and CVS carry a small risk of miscarriage. Results of some studies suggest that in a small number of cases, CVS may cause defects in the infant's fingers or toes. However, this only appears to occur if the test is done before the 9th week of pregnancy. Your doctor will talk to you about the risks and benefits of amniocentesis and CVS.
Written by familydoctor.org editorial staff