Chorionic villus sampling (CVS) is a prenatal test in which a sample of chorionic villi is removed from the placenta for testing. The sample can be taken through the cervix (transcervical) or the abdominal wall (transabdominal).

During pregnancy, the placenta provides oxygen and nutrients to the growing baby and removes waste products from the baby's blood. The chorionic villi are wispy projections of placental tissue that share the baby's genetic makeup. The test can be done as early as 10 weeks of pregnancy.

Chorionic villus sampling can reveal whether a baby has a chromosomal condition, such as Down syndrome, as well as other genetic conditions, such as cystic fibrosis. Although chorionic villus sampling can provide valuable information about your baby's health, it's important to understand the risks — and be prepared for the results.

Chorionic villus sampling can provide information about your baby's genetic makeup. Generally, chorionic villus sampling is offered when the test results might have a significant impact on the management of the pregnancy or your desire to continue the pregnancy.

Chorionic villus sampling is usually done between weeks 11 and 14 of pregnancy — earlier than other prenatal diagnostic tests, such as amniocentesis.

You might consider chorionic villus sampling if:

• You had positive results from a prenatal screening test. If the results of a screening test — such as the first trimester screen or prenatal cell-free DNA screening — are positive or worrisome, you might opt for chorionic villus sampling to confirm or rule out a diagnosis.
• You had a chromosomal condition in a previous pregnancy. If a previous pregnancy was affected by Down syndrome or another chromosomal condition, this pregnancy may be at a slightly higher risk, too.
• You're 35 or older. Babies born to women 35 and older have a higher risk of chromosomal conditions, such as Down syndrome.
• You have a family history of a specific genetic condition, or you or your partner is a known carrier of a genetic condition. In addition to identifying Down syndrome, chorionic villus sampling can be used to diagnose many other genetic conditions — including single gene disorders such as Tay-Sachs and cystic fibrosis.

Chorionic villus sampling can't detect certain birth defects, such as neural tube defects. If neural tube defects are a concern, an ultrasound or genetic amniocentesis might be recommended instead.

Your health care provider might caution against transcervical chorionic villus sampling — which is done through the vagina — if you have:

• An active cervical or vaginal infection, such as herpes
• Vaginal bleeding or spotting in the previous two weeks
• An inaccessible placenta, due to a tilted uterus or noncancerous growths in your cervix or the lower part of your uterus

Rarely, your health care provider might caution against transabdominal chorionic villus sampling — which is done through the abdominal wall — if:

• Your uterus is titled backward and your placenta is located at the back of your uterus

Chorionic villus sampling carries various risks, including:

• Miscarriage. The risk of miscarriage after chorionic villus sampling is estimated to be 0.22 percent.

• Rh sensitization. Chorionic villus sampling might cause some of the baby's blood cells to enter your bloodstream. If you have Rh negative blood and you haven't developed antibodies to Rh positive blood, you'll be given an injection of a blood product called Rh immune globulin after chorionic villus sampling.

  This will prevent your body from producing Rh antibodies that can cross the placenta and damage the baby's red blood cells. A blood test can detect if you've begun to produce antibodies.

• Infection. Very rarely, chorionic villus sampling might trigger a uterine infection.

Some older studies suggested that chorionic villus sampling might cause defects in a baby's fingers or toes. However, the risk appears to be a concern only if the procedure is done before week 10 of pregnancy.

You might need to have a full bladder for chorionic villus sampling. Check with your provider about how much fluid you might need to drink, as well as any other pre-test preparation that might be necessary before your appointment.

Your health care provider will explain the procedure and ask you to sign a consent form before the procedure begins. Consider asking someone to accompany you to the appointment for emotional support or to drive you home afterward.

Chorionic villus sampling is usually done in an outpatient facility or the health care provider's office.

During the procedure

First, your health care provider will use ultrasound to verify your baby's gestational age and the position of the placenta. You'll lie on your back on an exam table with your abdomen exposed. Your health care provider will apply a special gel to your abdomen, and then use a small device known as an ultrasound transducer to show your baby's position on a monitor.

Next, your health care provider will use the ultrasound image as a guide and take the tissue sample from the placenta while you lie still. This can be done through your cervix (transcervical) or your abdominal wall (transabdominal).

• Transabdominal chorionic villus sampling. After cleansing your abdomen with an antiseptic, your health care provider will insert a long, thin needle through your abdominal wall and into your uterus. You might notice a stinging sensation when the needle enters your skin, and you might feel cramping when the needle enters your uterus. The tissue sample from the placenta will be withdrawn into a syringe, and the needle will be removed.
• Transcervical chorionic villus sampling. After cleansing your vagina and cervix with an antiseptic, your health care provider will open your vagina with a speculum and insert a thin, hollow tube through your cervix. When the catheter reaches the placenta, gentle suction will be used to remove a small tissue sample.

After the procedure

You might experience a small amount of vaginal bleeding immediately after the procedure.

You can resume your normal activity level after the procedure. However, you might consider avoiding strenuous exercise and sexual activity for a day.

Meanwhile, the tissue sample will be analyzed in a lab. Results might take a few days to a week or so, depending on the complexity of the lab analysis.

Contact your health care provider if you have:

• Fluid leaking from your vagina
• Heavy bleeding
• A fever
• Uterine contractions

Your health care provider or a genetic counselor will help you understand your chorionic villus sampling results. Occasionally, test results are unclear and amniocentesis — another prenatal diagnostic test — is needed to clarify the diagnosis.

With chorionic villus sampling, there's a rare chance of a false-positive test — when the test is positive, but no disease exists. It's also important to remember that chorionic villus sampling can't identify all birth defects, including spina bifida and other neural tube defects.

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