Most women in the United States give birth to healthy babies.  Despite this, many women worry about birth defects. Some women are at higher risk than others for having a baby with a birth defect. While not all birth defects can be detected before delivery, certain tests may find some birth defects during pregnancy. Two such tests are amniocentesis and chorionic villus sampling (CVS). 

Some genetic problems can be found by studying the amniotic fluid that surrounds the fetus or the chorionic villi that make up the placenta (the tissue that connects the umbilical cord to the uterus). This can be done by amniocentesis or CVS. For these procedures, a sample is taken to be tested from either the amniotic fluid surrounding the fetus or the placenta. 

Amniocentesis and CVS can help identify chromosomal problems such as Down's syndrome or other genetic diseases such as cystic fibrosis, Tay-Sachs disease, and sickle cell disease. They can also be used to find out the sex of the fetus.

The following topics are covered below. You may click on a topic to go straight to that topic as well. 
  

Who Should Be Tested?

Your health care provider can tell you about your genetic risks and the testing available. Only you and your partner can decide whether to have a test. Each pregnancy is different, and there is some risk involved. Testing should be offered to: 

Normal test results cannot guarantee that a baby will be normal. There are several reasons why a birth defect may go undetected.  First, each test performed on the fetus looks for a specific problem. There may be a problem that the test was not designed or able to find. Second, some problems do not yet have a detection test.  Finally, no test is 100% accurate. 

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Amniocentesis

Amniocentesis is the most common procedure used to test for birth defects. It is usually done at 15-18 weeks of pregnancy. It is performed in a hospital or in the health care provider's office. You do not need to stay overnight. 
 

The Procedure 

A sample of amniotic fluid is withdrawn through a needle from the amniotic sac that surrounds the fetus. Amniotic fluid contains cells from the fetus that can be tested. These cells have the same genetic makeup as the fetus. 

For the procedure, the patient lies down and her abdomen is uncovered. First, ultrasound is used to show the health care provider where to insert the needle to avoid touching the fetus. The needle is then carefully guided through the abdomen and the uterus into the amniotic sac. A small sample (about 1 ounce) of fluid is withdrawn. If you are carrying twins, the health care provider will need to take a sample from each sac. The fetus will produce more amniotic fluid to replace the fluid that is removed. 

The amniotic fluid is sent to a lab. The cells are grown in a special fluid for several days. On the rare occasion that the cells don't grow, the procedure may need to be repeated. This does not mean that there is a problem with the fetus. The tests that are done with amniotic fluid cells depend on your own and your family's history. 
 

Results 

It may take about 2 weeks for enough cells to grow and tests to be performed. 

Tests of the amniotic fluid itself are another way to find some defects. One such test is the alpha-fetoprotein (AFP) test. AFP is a protein made by every growing fetus. Small amounts of AFP pass into the amniotic fluid. Too much AFP in the amniotic fluid can be a sign of fetal defects, such as open neural tube defects or openings in the fetus' abdomen. There is another AFP test that is a blood test. It checks the levels of AFP in the woman's blood. This test may help a woman decide whether to have amniocentesis. 

Complications 

Although amniocentesis is fairly safe, there is some risk involved. Side effects that may occur include cramping, bleeding, infection, leaking of amniotic fluid after the procedure, and miscarriage--less than 1 in 200 women who have the test will have a miscarriage that they would not have had otherwise.  Injury to the fetus from amniocentesis is rare. 

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Chorionic Villi Sampling

CVS can be performed earlier in pregnancy than amniocentesis. It is usually done about 10-12 weeks from the woman's last menstrual period. This allows for earlier detection of birth defects. Like amniocentesis, CVS is performed in a hospital or the health care provider's office. CVS is a somewhat newer test than amniocentesis and is not available in all areas. 

The Procedure  

When CVS is performed, a small sample of cells is taken from the placenta where it is attached to the wall of the uterus.  Chorionic villi are tiny parts of the placenta. Villi are formed from the fertilized egg, and have the same genes as the fetus. 

There are two ways to collect cells from the placenta:  through the vagina or through the abdomen. To collect cells through the vagina, a speculum is inserted just as for a Pap test. Then, a very thin, plastic tube is inserted into the vagina and up through the cervix. With ultrasound, the tube is guided to the placenta. A small sample is removed. To collect cells through the abdomen, a slender needle is inserted through the woman's abdomen to the placenta, much like amniocentesis. The sample of chorionic villi is then sent to a lab. There the cells are grown and tested. 
 
If you have an active sexually transmitted disease, bleeding during pregnancy, or certain problems with the cervix, you may be offered CVS, but only through the abdomen. In other cases, neither type of CVS may be the best choice. 

Results 

CVS can detect most of the same defects as amniocentesis. One defect that cannot be detected by CVS is open neural tube defects.  If you have CVS you may want to consider having a blood AFP test later in the pregnancy to screen for neural tube defects. The results of CVS can be obtained earlier in pregnancy and more quickly than with amniocentesis. Most women get their results within 10 days. 

Complications

CVS may carry a slightly higher risk of miscarriage than amniocentesis. The rate is higher than that for amniocentesis because CVS is done earlier. Infection can also be a complication of CVS. Rare cases of limb deformities in infants have been reported, especially when CVS was done before 10 weeks.  

Options 

If you have certain risk factors, you may be offered amniocentesis or CVS to try to detect certain birth defects.  Whether you have the test done is up to you. Some people choose not to get this information. 

Most of the time, tests show normal results, which reduce parents' fears and anxieties. If your tests diagnose a major birth defect, you have difficult decisions to make. You may choose to continue the pregnancy and have the baby, or you may choose to terminate the pregnancy. If you choose to continue the pregnancy, you may need to deliver your baby at a special hospital. You may also need extra help after the baby is born. The earlier decisions are made, the more time you will have to seek professional support and necessary medical care.

When making your decision, you should get as much information about the defect as you can – from health care providers, counselors, or parents of a child with the same condition. Ask friends or family for advice and support. Knowing as much as possible will help you to make the best decision.

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