Topic Overview

What are birth defects tests?

Birth defects tests are done during pregnancy to look for possible problems with the baby (fetus). Birth defects develop when something is wrong with genes or chromosomes, an organ, or body chemistry. A birth defect may have only a mild impact on a child's life, or it can have a major effect on quality of life or life span.

Birth defects include:

• Genetic disorders, such as Down syndrome and trisomy 18.
• Diseases passed in a family, such as Tay-Sachs, sickle cell anemia, and cystic fibrosis.
• Structural problems, such as heart defects and neural tube defects, including spina bifida.

What are the types of tests?

There are two types of birth defects tests: screening and diagnostic.

• Screening tests show the chance that a baby has a certain birth defect. It can't tell you for sure that your baby has a problem. If the test result is "positive," it means that your baby is more likely to have that birth defect. So your doctor may want you to have a diagnostic test to make sure. If the screening test result is "negative," it means that your baby probably doesn't have that birth defect. But it doesn't guarantee that you will have a normal pregnancy or baby.
• Diagnostic tests show if a baby has a certain birth defect.

Screening tests for birth defects are blood tests and ultrasounds. The blood tests are used to look for the amount of certain substances in your blood. The doctor uses an ultrasound to look for certain changes in the baby. Diagnostic tests involve taking some of the baby's cells to look at the genes and chromosomes.

No test is 100% accurate. A test may be negative even when the baby has a birth defect. This is called a false-negative test result. It's also possible that a test will be positive—meaning the test result is abnormal—but the baby does not have the problem. This is called a false-positive test result.

You may have only first-trimester tests or only second-trimester tests. Or you may have an integrated test. This test combines the results of tests you have in your first trimester and second trimesters.

Should you have birth defects tests?

Pregnant women and their partners can choose whether to have a test for birth defects. For example, you may want to have tests to know if there is a problem so you can work with a doctor and hospital to care for your baby after birth. Or you may want to have tests because you wouldn't want to continue the pregnancy if there is a serious problem. Some women might decide not to have these tests because they would continue the pregnancy regardless of the results.

Talk to your doctor about tests that are available where you live and which tests might be best for you.

If you choose to have a test, you also may want to talk with a genetic counsellor. The counsellor can talk with you about the reasons to have or not have the test. He or she can also help you find other resources for support and decision-making.

Deciding about testing
can be a hard and emotional choice. You need to think about what the results of a test would mean to you and how they might affect your choices about your pregnancy.

Types of Tests

You and your doctor can choose from several tests. What you choose depends on your wishes, where you are in your pregnancy, your family health history, and what tests are available in your area. You may have no tests, one test, or several tests.

Screening tests show the chance that a baby has a certain birth defect. Diagnostic tests show if a baby has a certain birth defect.

First-Trimester Tests

Screening tests

The nuchal translucency test and the first-trimester blood tests are often done together in what is called the first-trimester screening.

• Nuchal translucency test. This test uses ultrasound to measure the thickness of the area at the back of the baby's neck. An increase in the thickness can be an early sign of certain birth defects, such as Down syndrome. This test is often done along with blood tests in the late first trimester. It is not available everywhere, because a doctor must have special training to do it.
• First-trimester blood tests. These tests measure the amounts of two substances in your blood: beta human chorionic gonadotropin (beta-hCG) and pregnancy-associated plasma protein A (PAPP-A). Beta-hCG is a hormone made by the placenta. High levels may be related to certain birth defects. PAPP-A is a protein in the blood. Low levels may be related to certain birth defects. The doctor looks at the test results—along with your age and other factors—to find out the chance that your baby may have certain problems.

First-trimester tests also can be done as part of an integrated screening test. This test combines the results of the first trimester tests (first-trimester blood tests and nuchal translucency test) with those of a second trimester test (the triple or quad screening).

Cell free fetal DNA

The cell free fetal DNA test looks at fetal DNA in a pregnant woman's blood. It can help find genetic problems like Down syndrome or trisomy 18. This test is an option for women who have risk factors for having a baby with certain birth defects. It's not used as a general screening test or for women carrying twins.

Diagnostic test

• 
  Chorionic villus sampling (CVS)
  . Doctors can use this test to look at cells in the placenta. CVS can be done between 10 and 13 weeks of pregnancy. A doctor collects a sample of chorionic villus cells by putting a thin flexible tube (catheter) into your uterus through your vagina or by putting a needle through your belly into your uterus. The test can be used to find chromosomal birth defects such as Down syndrome and family diseases such as sickle cell disease or cystic fibrosis. But it cannot find neural tube defects.

Second-Trimester Tests

Screening tests

• 
  Triple or quadruple (quad) blood tests
  . These tests check the amounts of three or four substances in a pregnant woman's blood. The triple screen checks the levels of alpha-fetoprotein (AFP), beta human chorionic gonadotropin (beta-hCG), and a type of estrogen(estriol, or uE3). The quad screen checks those three substances, plus the level of the hormone inhibin A. The doctor looks at these test results—along with your age and other factors—to find out the chance that your baby may have certain problems. Second-trimester tests can be done between 15 and 20 weeks of pregnancy. The triple and quad blood tests may be called the expanded AFP test, the AFP plus test, or the multiple marker screening test. Either of these tests may be done as the second part of the integrated screening test.
• 
  Ultrasound
  . This test allows your doctor to see an image of your developing baby. It is often done at 18 to 20 weeks of pregnancy. Doctors can use ultrasound to look for certain features that are related to some problems, such as Down syndrome. The test also can be used to find problems of the heart, spine, belly, or other areas.

Diagnostic test

• 
  Amniocentesis
  . Doctors use this test to look for chromosomal problems in the baby's cells. A doctor puts a needle through the belly and into the uterus to collect some of the amniotic fluid that surrounds the baby. This fluid contains some of the baby's cells. The test is done between 15 and 20 weeks of pregnancy, usually around week 16. This test also can help find neural tube defects, such as spina bifida.

Deciding About Testing

The decision to have a test for birth defects is personal. You have to think about your age, your chance of passing on a family disease, your need to know about any problems, and what you might do after you have the test results. Your spiritual beliefs and other values also may play a role in your decision.

Some birth defects—such as a cleft lip or cleft palate or certain heart problems—can be fixed with surgery after birth and sometimes even with surgery during pregnancy. Some other defects cannot be fixed.

Reasons to have tests

You may decide to have the tests because:

• You would think about whether to continue the pregnancy if there is a birth defect.
• You have a family history of an illness such as cystic fibrosis, Tay-Sachs disease, or hemophilia. To learn more about cystic fibrosis testing, see the topic Cystic Fibrosis Carrier Screening.
• Your provincial health plan or private health insurance will pay for some or all of the cost of the tests.
• You need to know whether there might be a problem, because not knowing would make you worry.
• You want to learn all you can about caring for and raising a child with a certain birth defect.
• You would want to plan what hospital and doctors to work with to make sure that all of your baby's needs are met.

Reasons NOT to have tests

You may decide not to have birth defects tests because:

• You would continue the pregnancy no matter what the tests show.
• The screening test may show an increased risk for a problem even though your baby doesn't have the problem. You would be worried while you wait for the test results.
• These tests can't find all possible problems.
• You are worried about the risk of miscarriage. Both CVS and amniocentesis have a small risk of miscarriage.
• Tests can be expensive. You may not have insurance, or your insurance may not pay for the tests.

For more help on deciding about tests, see the topics:

• Pregnancy: Should I Have Screening Tests for Birth Defects?
• Pregnancy: Should I Have CVS (Chorionic Villus Sampling)?
• Pregnancy: Should I Have Amniocentesis?