You may be asked to drink a glass of fluid about an hour before the test so that your bladder is full. A full bladder makes it easier to do the test.

You may want to talk to a genetic counsellor before or after the test. This person is trained to give you detailed information about the test. The counsellor can help you make decisions about testing. The counsellor can also help you understand the results of the tests.

You will be asked to sign a consent form that says you understand the risks of the test and agree to have it done.

CVS is usually done by a perinatologist, Opens dialog. It may be done in the doctor's office or the hospital. An overnight stay in the hospital usually is not needed.

CVS may be done either through the belly (transabdominal) or through the cervix, Opens dialog (Figure 4) (Figure 4) (transcervical). The choice may depend on the position of the fetus and the placenta.

Through the belly (transabdominal)

You will lie on your back on an examination table with your belly bare.

Ultrasound will be used to help your doctor guide the needle to the placenta. Gel will be rubbed on your belly. An ultrasound device (transducer) will be moved over your belly. It gives off and picks up sound waves. The sound waves are sent to a computer to make a picture of your uterus, Opens dialog, your baby, and the placenta on a screen. The doctor can also use ultrasound to check your baby's heart rate.

Your skin where the needle will be put in is cleaned with an antiseptic and may be numbed with a medicine. Your doctor then puts a long, thin needle through your belly and uterus to the placenta. The needle is used to collect a sample of the chorionic villus cells.

After the sample is collected, the doctor may listen to your baby's heart and check your blood pressure, pulse, and breathing.

Through the cervix (transcervical)

You will take off your clothes below the waist. You will have a paper or cloth covering around your waist. You will lie on your back on an examination table with your knees bent and feet and legs supported by footrests.

Your doctor will put a special tool with curved sides (speculum) into your vagina. The speculum helps to open the vagina a little bit. This lets the doctor see the inside of the vagina and the cervix. The cervix will be cleaned with an antiseptic.

Your doctor will use ultrasound to guide a thin tube (catheter) through your cervix to the placenta. Gel will be rubbed on your belly. An ultrasound device (transducer) will be moved over your belly. It gives off and picks up sound waves. The sound waves are sent to a computer to make a picture of your uterus, your baby, and the placenta. The doctor can also use ultrasound to check your baby's heart rate.

When the catheter is in the right place, a sample of chorionic villus cells will be collected.

After the sample is collected, the doctor may listen to your baby's heart and check your blood pressure, pulse, and breathing.

How long the test takes

The test will take about 30 minutes. Getting the sample takes only a few minutes.

Through the belly (transabdominal)

You may feel a short, sharp sting from the needle used to give the numbing medicine. There is usually no pain when the collecting needle is put in the belly. You may have some cramping when the needle is in your uterus.

Through the cervix (transcervical)

Most women do not find this procedure painful. You may feel some pressure or mild discomfort when the speculum is placed in your vagina. You may have some cramping when the catheter is put through your cervix.

Some risks of CVS may include:

• Infection in your uterus.
• Vaginal bleeding.
• Leakage of amniotic fluid.
• Miscarriage. About 1 out of 500 women have a miscarriage after this test.

There is a very small chance that CVS could cause bleeding that could let your blood mix with your baby's blood. If you have Rh-negative blood, Opens dialog, you will be given the Rh immunoglobulin shot (such as WinRho). This can prevent Rh sensitization, Opens dialog, which could cause harm if your baby has Rh-positive blood.

The genetic material (chromosomes, Opens dialog) of the cell sample is looked at carefully. Normally there are 46 chromosomes in each cell, arranged in 23 pairs. The number and arrangement of the chromosomes in the sample can show if your baby has a genetic disease. Chromosomes also show the sex of your baby.

It may take 1 to 2 weeks to get CVS results.

CVS can find certain problems, but normal results don't guarantee that your baby will be healthy.

In rare cases, the chorionic villus cells may have abnormal genetic material even though the baby's cells are normal. This is called a false-positive result, Opens dialog. An abnormal test result may need to be confirmed by amniocentesis.