About Birth Defects
About 3 percent of babies born in the United States have some type of major birth defect.

A baby may be born with more than one birth defect.

In most cases, there is no known reason for a birth defect. In some cases, birth defects are inherited. Others can occur if the fetus is exposed to certain drugs (including alcohol), chemicals or viruses during key stages of growth during pregnancy.

Genetic Problems
Genes are passed by parents to their newborn through tiny structures called chromosomes. Chromosomes are present in every human cell. The mother's egg and the father's sperm each contain 23 chromosomes. When the sperm joins the egg, the genes and chromosomes of both parents unite to form those of the fetus.

Thus, an error in a chromosome or a single gene may result in birth defects.

A birth defect can be passed from the parents to their baby through their genes. This is called a genetic disorder.

A defect also may result from problems with the number or structure of whole chromosomes. There may be too many or too few chromosomes. In Down syndrome, for instance, there is an extra (third) copy of one of the chromosomes.

Some genetic problems can be found in two ways:

• Amniocentesis — studying the amniotic fluid that surrounds the fetus
• CVS — studying the chorionic villi that make up the placenta

For these procedures, a sample is taken to be tested from either the amniotic fluid that surrounds the fetus or the placenta.

Who Should Be Tested?
Your doctor can tell you about your genetic risks and the tests you can have. Only you and your partner can decide whether to have a test.

Testing should be offered to:

• Pregnant women who will be 35 or older on their due date
• Couples who already have had a child with a birth defect or have a family history of certain birth defects
• Pregnant women with other abnormal genetic test results

A normal test result on the fetus cannot ensure that the baby will be normal.

Amniocentesis
Amniocentesis is the most common procedure used to test for birth defects. It is done at 16 to 18 weeks of pregnancy in most cases.

The Procedure
With amniocentesis, a sample of amniotic fluid is withdrawn through a needle from the sac that surrounds the fetus.

The amniotic fluid is sent to a lab. The cells are grown in a special fluid for several days. Then, tests are done.

Results
It may take about two 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. Too much AFP in the amniotic fluid can be a sign of fetal defects, such as open neural tube defects or openings in the fetal abdomen.

One type of AFP test is a blood test. It checks the levels of AFP in the woman's blood.

Risks
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
• Miscarriage

Injury to the fetus from amniocentesis is rare.

Chorionic Villus Sampling
CVS can be done earlier in pregnancy than amniocentesis. In most cases, it is done about 10 to 12 weeks from the woman's last menstrual period.

The Procedure
With CVS, a small sample of cells is taken from the placenta where it is attached to the wall of the uterus.

Results
CVS can detect most of the same defects as amniocentesis. One defect that cannot be detected by CVS is open neural tube defects.

The results of CVS can be obtained earlier in pregnancy and more quickly than with amniocentesis.

Risks
CVS may carry a slightly higher risk of miscarriage than amniocentesis.

Advantages of Each Method
Amniocentesis and CVS both have advantages. With amniocentesis:

• It often is easier to have done because it is more widely available.
• It offers a lower risk of miscarriage than CVS.
• It can test for neural tube defects because it tests the amniotic fluid.

On the other hand, CVS can be done earlier in pregnancy.

Options
Most of the time, tests show normal results, which reduce patients' fears and anxieties. If your tests diagnose a major birth defect, you have tough choices to make.

Before you decide, get as much information about the defect as you can — from doctors, counselors or parents of a child with the same type of defect. Ask friends or family for advice and support. Knowing as much as you can will help you to make the best choice.

Finally …
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. Keep in mind that in most pregnancies, babies are born healthy.

This excerpt from ACOG's Patient Education Pamphlet is provided for your information. It is not medical advice and should not be relied upon as a substitute for visiting your doctor. If you need medical care, have any questions, or wish to receive the full text of this Patient Education Pamphlet, please contact your obstetrician-gynecologist.