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What is prenatal diagnosis?

Prenatal diagnosis means diagnosis before birth. Prenatal diagnosis is a way for your healthcare provider to determine, before birth, whether your fetus has a problem such as Down syndrome. Amniocentesis and chorionic villus sampling (CVS) are tests that help find genetic disorders before birth.

Some parents are at increased risk of having a baby with a genetic disorder or other problem and may want to have one of these tests. Knowing about problems before the baby is born may help parents make decisions about health care for their infant. Certain problems can be treated before the baby is born, while other problems may need special treatment right after delivery. In some cases, parents may decide not to continue the pregnancy.

Parents aren’t required to have amniocentesis and chorionic villus sampling. Discuss the options with your healthcare provider.

 Path to improved health

Amniocentesis or CVS is done when there is an increased risk that the baby may have genetic disorders or birth defects. It’s often done if:

• You are 35 years of age or older by the time your baby is due. People in this age group have increased risk of having a baby with a chromosome abnormality. This could include Down syndrome.
• You had a screening test that showed there could be a problem.
• You have had a child with Down syndrome, spina bifida, or other disorder.
• You or your partner is a known carrier of a genetic disorder, such as cystic fibrosis.

Before either procedure, you may have counseling with a genetic expert. This allows you to learn about the conditions the test may find. You will have a better idea of what those mean for you and your baby.

How is amniocentesis performed?

In this procedure, a sample of amniotic fluid (the fluid around the baby) is removed from your uterus and sent to a laboratory for evaluation. Amniocentesis is performed by inserting a thin needle through your abdomen into your uterus (womb) and withdrawing a small amount of fluid. The fluid is sent to a laboratory for evaluation. In the lab, the fluid can be tested for:

• Genetic abnormalities
• Infection
• Signs of lung development

Your body will make more fluid to replace the fluid that is taken out. The baby will not be hurt during the procedure. You may feel mild cramping during or after the procedure. Your healthcare provider may tell you to rest on the day of the test, but usually you can resume normal activity the next day.

How is CVS performed?

CVS removes a small sample of the placenta tissue from the uterus. The sample can be taken 2 ways:

• A catheter (a thin tube) through the vagina.
• A needle through the abdomen. and the needle then withdraws placenta tissue.

Most people feel fine after the test, although some may have mild bleeding (spotting) afterward.

When are the tests performed?

Amniocentesis is usually performed after the 15^th week of pregnancy. CVS is usually performed between the 11th and 14th weeks of pregnancy.

Is one test better than the other?

The main benefit of CVS is that it can be done earlier in the pregnancy. It’s very accurate in detecting genetic abnormalities. But it does not detect some things that amniocentesis does. These include:

• Neural tube defects (involving the spinal column or brain).
• Birth defects.
• Rh incompatibility (when the mother has Rh-negative blood and the baby has Rh-positive blood).

Amniocentesis might be the best option if:

• You previously had a baby with a neural tube defect.
• You or your partner have a neural tube defect.
• The results of other tests during your pregnancy have been abnormal. This could include a blood screening test done in early pregnancy.

CVS may be better if you and your healthcare provider want to know the test results during your first trimester.

 Things to consider

Amniocentesis and CVS carry a few small risks.  They include:

• Miscarriage
• Infection or injury to the baby
• Leaking of amniotic fluid
• Vaginal bleeding

The risks of CVS are slightly higher than for amniocentesis. Your healthcare provider will talk to you about the risks and benefits of amniocentesis and CVS.

FOR MORE INFORMATION

IWK Health

https://iwkhealth.ca/pregnancy/prenatal-screening

Public Health Agency of Canada at:

 https://www.canada.ca/en/public-health/services/fertility/genetic-testing-screening.html