Diagnostic tests can provide early detection of birth defects

At some point in pregnancy, almost all pregnant women lie awake at night wondering: "Will my baby be normal? Will she have a problem?" The truth is that most women in the United States give birth to healthy babies. However, for women whose family history, genetics, age or other factors put them at higher risk of having a baby with a birth defect, prenatal diagnostic tests are a must.

Each test assesses for particular problems. However, there can be problems that aren't detected, and no test is 100% foolproof.

Some couples choose to get prenatal testing on the assumption that they will terminate the pregnancy if they discover that their baby has a serious birth defect. Other couples who would not choose abortion still choose to get tested, so that they can better prepare themselves should their baby be born ill or with special needs.

Three common tests to know about include chorionic villus sampling (CVS), amniocentesis, and alpha-fetoprotein.

Amniocentesis studies amniotic fluid

Amniocentesis is the most common procedure used to test for birth defects. It is usually offered at about 15 weeks into a pregnancy to:

Pregnant women who are age 35 or older. This is because, as a woman's age increases, so does her risk of having a child with a chromosomal problem such as Down's Syndrome.
Couples who already have a child with a birth defect or a family history of particular birth defects.
Pregnant women with other abnormal genetic test results.

You won't need to stay overnight at the hospital. The test is usually done in a hospital clinic or your doctor's office. First, you'll have an ultrasound so that your doctor can see where to insert the needle safely. Then, your doctor will insert a very thin needle through your abdomen into the amniotic sac and withdraw cell-rich fluid that can be tested. The fluid is then sent to a lab and tests are done.

It may take about two weeks to get test results. What tests are ordered depends on your special circumstances.

Although amniocentesis is fairly safe, there are some risks to the fetus. Side effects that can occur include:

Cramping
Bleeding
Infection
Leaking of amniotic fluid
Miscarriage (The risk from amniocentesis alone is very slight.)

CVS is done earlier than amnio

Chorionic Villus Sampling (CVS) is usually offered for the same reasons and detects most of the same defects as amniocentesis (although it doesn't detect neural tube defects, which amniocentesis can). It is performed at about 10 to 12 weeks after your last menstrual period.

With CVS, a small sample of placental cells is studied. The cells are collected either through the vagina with a speculum (the same instrument used for a pap test) or through the abdomen in a procedure much like amniocentesis.

Results are usually available in 10 days. The procedure is slightly more risky than amniocentesis. The advantage to CVS is that it's done earlier than amniocentesis. However, in some parts of the United States the procedure isn't as readily available as amniocentesis, so you may have to travel to a major medical center to have it done.

Alpha-fetoprotein detects neural tube defects

The alpha-fetoprotein (AFP) test is a blood test usually given at or between 16 and 18 weeks of pregnancy. An AFP test checks the levels of this protein in the mother's blood. Too much of the protein can signal a birth defect, specifically, a neural tube defect (problems in which the brain or spinal cord do not form properly). Too low an amount of the protein may be linked to an increased risk of Down's Syndrome. The AFP blood test does not expose you to risk of miscarriage.

Date last reviewed: October 2002.

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