CVS (chorionic villus sampling) is a test that is done to see if your baby may be born with certain kinds of serious health problems.

CVS is usually done between weeks 10 and 13 of a woman's pregnancy. But it's not a routine test. Your doctor may recommend it if a fetal ultrasound and blood tests suggest that your chances of having a baby with a genetic condition or birth defect are higher than average. Or you may already know that your chances are higher because of your age and family history.

Chorionic villi are tiny finger-shaped growths in the placenta. The genetic material in these cells is the same as that in the baby's cells. Early in your pregnancy, a doctor can take a sample of these cells and check them for certain health problems.

Chorionic villus sampling can tell you if your baby may be at risk for having:

• A genetic condition, such as Down syndrome.
• A disease that can run in families, such as Tay-Sachs disease or hemophilia.

Even if the results from your CVS are normal, it doesn't guarantee that your baby will be born healthy. No test can do that. For example, CVS can't find many common problems, such as defects of the heart, stomach, intestines, or brain and spine (neural tube defect). A blood test (alpha-fetoprotein test) may be done early in your second trimester to check for some of these defects.

There is another test that can be done during your second trimester that looks for both genetic conditions and neural tube defects. This test is called amniocentesis. It's usually done between weeks 15 and 20 of your pregnancy.

Some women choose to wait until they can have amniocentesis. Others choose to have CVS in their first trimester. If it shows a serious problem, they have more time to make decisions about their pregnancy.

Chorionic villi cells can be collected in one of two ways. During the test, your doctor may:

• Put a thin, flexible tube called a catheter through your vagina and cervix into the placenta. This is called transcervical CVS.
• Put a long, thin needle through your belly into the placenta. This is called transabdominal CVS.

The doctor uses ultrasound to guide the catheter or needle to the right spot.

The test can tell you if your baby may be born with certain kinds of serious health problems. Many parents are not prepared to care for a baby who is sick or has a birth defect. Information that you get from this test can help you and your partner plan for the future.

If the test finds that your baby has a genetic condition or a birth defect, you and your partner may be faced with a tough decision about whether to continue the pregnancy. It may be helpful to talk with your doctor and a genetic counsellor. They can help you understand your baby's health problem and what to expect when he or she is born.

Results from the test can also help you decide where to have your baby. If your baby will need surgery or special care, you can plan to have your baby in a hospital that has special services for newborns, such as a neonatal intensive care unit.

A CVS test has some risks. You'll have to weigh the benefit of knowing if something might be wrong with your baby against the risks of having the test.

There is a small chance that the test may cause you to have a miscarriage. This means that you could lose your baby after you have the test. But when the test is done by highly trained doctors, the risk of having a miscarriage is small.

• In studies of women who had CVS performed by a highly trained doctor, about 1 out of 455 women had a miscarriage after the test, while 454 did not.^{footnote 1}

Other risks include:

• Infection. There is a chance that you could get an infection in your uterus.
• Bleeding. There is a very small chance that you could bleed during the test. If this happens, your blood may mix with your baby's blood. This is only a problem if your blood is Rh-negative and your baby's is Rh-positive, because you could have an immune system response called Rh sensitization. If you're at risk for Rh sensitization, you'll be given a vaccine to prevent it.
• Arm or leg defect. There is a chance that your baby may be born with an arm or leg defect. But this is rare, especially if the test is done after week 10 of your pregnancy.

Before you decide to have CVS, you might think about:

• Your chance of passing on a family disease to your baby.
• Your age. As you get older, you have a greater chance of having a baby with a birth defect.
• Your need to know about any problems with your baby.
• What you might do if the test shows a problem.
• Whether you can afford to pay for the test. CVS can cost a lot, and the test is not offered in all places. Most provincial health plans and private insurance plans will cover the cost of the test if you have certain risk factors that may increase your baby's chance of having a serious health problem. A risk factor is something—such as your age or family history—that raises your risk of having a certain health problem.

Your doctor may advise you to have CVS if:

• Other tests suggest that your chance of having a baby with a genetic condition or birth defect is higher than average.
• You or your partner has a family history of genetic conditions or birth defects.
• You or your partner carries an abnormal gene that is known to cause a certain disease.
• You already have a child who has Down syndrome or another chromosomal defect.
• You want to know if your baby has a serious health problem so you can decide early whether you want to continue your pregnancy or make plans to care for a sick child.