Prenatal testing can provide valuable information about your baby's health. But early diagnostic tests have potentially serious risks as well. If you're considering chorionic villus sampling, here's what you need to know.

Chorionic villus sampling (CVS) is a prenatal test used to identify various genetic problems, including Down syndrome. Using a thin tube guided through your cervix or a needle inserted into your uterus, your health care provider takes a sample of chorionic villi from the placenta. These wispy projections from the placenta — which have the same genetic makeup as your baby — transfer nutrients, oxygen and antibodies from you to your baby.

Chorionic villus sampling can provide genetic information about your baby earlier in your pregnancy than can other diagnostic tests, such as amniocentesis. The test might be offered if your baby has an increased risk of a specific chromosomal or genetic disorder. In other cases, chorionic villus sampling may be recommended if the results of a screening test in the first trimester cause concern.

Chorionic villus sampling cannot detect neural tube defects, such as spina bifida. If neural tube defects are a concern, an ultrasound or genetic amniocentesis may be recommended instead.

Chorionic villus sampling is usually done between the ninth and 14th weeks of pregnancy. Amniocentesis is typically done after the 15th week of pregnancy.

Chorionic villus sampling can be done in the doctor's office. The procedure begins with an ultrasound to determine the position of the placenta. Then the tissue sample is taken through the cervix (transcervical) or the abdominal wall (transabdominal).

• Transcervical CVS. If the placenta is in a favorable position, your health care provider may take the sample through your cervix. After cleansing your vagina and cervix with an antiseptic, he or she will open your vagina with a speculum and insert a thin, hollow tube (catheter) through your cervix. When the catheter reaches the placenta, gentle suction will be used to remove a small tissue sample. You may feel cramping during the procedure.
• Transabdominal CVS. If the placenta isn't clearly accessible through the cervix or you have a cervical infection, such as chlamydia or herpes, your health care provider may take the sample through a needle inserted into your uterus. After cleansing your abdomen with an antiseptic, he or she will insert a long, thin needle through your abdominal wall and into your uterus. The tissue sample from the placenta will be withdrawn into a syringe, and the needle will be removed.

Results may be available within two to seven days, depending on the complexity of the lab analysis.

Analysis of fetal cells can reveal whether your baby has a chromosomal abnormality, such as Down syndrome. Chorionic villus sampling can also be used to test for other genetic disorders, such as Tay-Sachs disease — but only if there is a specific reason to test for these conditions.

With chorionic villus sampling, the chance of a false-positive — when the test is positive, but no disease exists — is less than 1 percent. But chorionic villus sampling can't identify all birth defects, including spina bifida and other neural tube defects.

Chorionic villus sampling carries various risks, including:

• Miscarriage. Chorionic villus sampling has a one in 100 risk of miscarriage.
• Cramping and vaginal bleeding. You may feel cramping after the test. Vaginal bleeding also is possible, especially if the cell sample was taken through your cervix. If you develop heavy bleeding or a fever after chorionic villus sampling, contact your health care provider.
• Rh sensitization. Chorionic villus sampling may cause some of the baby's blood cells to enter your bloodstream. If you have Rh-negative blood and your baby has Rh-positive blood, you'll be given a drug called Rh immunoglobulin after the test to prevent you from producing antibodies against your baby's blood cells.

In the past, the risk of limb deformities was thought to be increased by chorionic villus sampling, but researchers have found no evidence to justify the initial fear. Today, chorionic villus sampling is not considered a risk factor for congenital defects of the limbs.

Chorionic villus sampling is recommended when the potential value of early diagnostic results outweighs the risk of miscarriage or other complications. Ultimately, the decision to have chorionic villus sampling is up to you. Your health care provider or genetic counselor can help you weigh all the factors in the decision.

Most test results are normal, which can ease anxiety about your baby's health. Sometimes a follow-up ultrasound is recommended several days after chorionic villus sampling to verify the baby's well-being. Rarely, CVS results are unclear and amniocentesis is needed to clarify the diagnosis.

Early diagnosis of certain disorders may lead to early treatment. For example, if a female baby has congenital adrenal hyperplasia — a condition in which excessive amounts of male hormones are produced — hormone therapy can be given to the mother to prevent the baby from developing male characteristics.

If chorionic villus sampling indicates that your baby has a chromosomal problem or a genetic disorder that can't be treated, you may be faced with wrenching decisions — such as whether to continue the pregnancy. Seek support from your health care team, your loved ones and other close contacts during this difficult time.