Prenatal Genetic Testing

What is prenatal genetic testing?

Prenatal genetic testing gives parents information about genetic disorders or birth abnormalities the fetus may have. Unlike some of the routine prenatal tests like blood type, blood count or glucose screening, these tests are optional. Talk with your healthcare provider and decide which tests are right for you.

Some people decide to get prenatal genetic testing in addition to routine pregnancy testing. Prenatal genetic testing is optional. A person’s chromosomes or genes cause genetic disorders. A gene is part of your body’s cells that stores instructions for how your body works. A chromosome holds your genes. Congenital disorders are present at birth, and healthcare providers can diagnose some of these conditions before birth.

There are two different kinds of tests: screening tests and diagnostic tests.

What types of prenatal screening tests may be available?

Screening tests don’t diagnose genetic conditions, and an abnormal result doesn’t mean the fetus has a condition. It means there’s a higher risk for a genetic disorder. Your healthcare provider can help interpret results from genetic screening tests and explain the next steps. In some cases, they may recommend diagnostic testing.

Some examples of prenatal genetic screening tests are:

Carrier screening

Carrier screening is a blood test that pregnancy care providers may offer you and your partner. This type of testing can identify small changes in the genes that increase the risk that the fetus could inherit serious medical conditions. These are often called single-gene conditions. Examples include cystic fibrosis, sickle cell disease and spinal muscular atrophy. If your blood test shows you’re a carrier for a genetic risk, your partner should also undergo screening. If both parents are carriers of the same genetic risk, the fetus could have a more severe form of the disease. You only need to complete carrier screening tests once in your lifetime.

Screening for an abnormal chromosome number

Chromosomes should be inherited in pairs — one half of the pair from each parent. Sometimes, nature makes mistakes during fertilization, resulting in missing, or extra, parts of the chromosome pair. Examples include Down syndrome (an extra chromosome 21) and Turner’s syndrome (a missing X chromosome). These test results will be different for each pregnancy. Types of tests include:

• Cell-free fetal DNA screening: This is popularly known as non-invasive prenatal testing (NIPT). Providers look for some common forms of abnormal chromosome numbers in small bits of fetal DNA found in your blood. Because fetal DNA is present in very small amounts, healthcare providers can’t perform the test until 10 weeks gestation.
• Serum screening: This type of test is also done on your blood sample. It doesn’t look directly at fetal DNA. Instead, these tests analyze various protein levels in your blood to determine the risk for abnormal chromosome numbers. Examples of this type of test include sequential screening, quad screening and first trimester serum screening. Each of these tests must occur at very specific times during pregnancy, so check with your provider about which ones might be right for you. Your healthcare provider can’t perform any of these tests until the 11th week of pregnancy.

Screening for physical abnormalities

Sometimes, missing or extra chromosomes cause changes in your baby’s physical structure. For example, the chromosomes are normal but your baby has a physical defect. During pregnancy, ultrasound and blood tests can help determine the fetus’s risk for physical abnormalities and assess whether or not they may be due to genetic abnormalities.

• Nuchal translucency: Uses ultrasound to measure the thickness of the back of the fetus’s neck. A thick nuchal translucency gives us clues about the risk for abnormal chromosome number and for physical abnormalities like abnormal fetal heart formation. Providers perform this ultrasound at 11 to 14 weeks gestation.
• AFP screening (maternal serum screen): Your provider draws some of your blood to measure the level of AFP in your blood. High levels may indicate physical problems with the fetus’s abdomen, face or spine. This occurs at 15 to 22 weeks gestation.
• Quad screen: Measures the levels of four substances in your blood and determines the risk of the fetus having chromosomal abnormalities and neural tube defects. This test is also called a multiple marker screen. It’s also done at 15 to 22 weeks gestation.
• Fetal anatomy scan: Uses ultrasound to evaluate the fetus’s physical structures, including the developing brain, skeleton, heart, kidneys, abdomen, face, arms and legs. This ultrasound is usually done at 18 to 20 weeks gestation.

Remember, prenatal screenings tell you a condition might be present. It doesn’t diagnose a condition.

What types of prenatal diagnostic tests are there?

Diagnostic prenatal tests can confirm if the fetus has a genetic condition. These tests take cells from the amniotic fluid or placenta and test them for specific disorders. Diagnostic testing is only performed when a screening test is abnormal or if you’re at high risk for having a baby with a genetic condition.

The most common diagnostic tests are amniocentesis and chorionic villus sampling (CVS).

• Amniocentesis: Your provider inserts a needle through your skin into your uterus to remove a sample of amniotic fluid from the amniotic sac. Amniocentesis happens between 16 and 20 weeks of pregnancy.
• CVS: Your provider inserts a needle into your uterus to remove a small sample of cells from the placenta. The provider will determine if it’s safer to place the needle through your abdomen or your vagina. CVS occurs around 11 to 13 weeks of pregnancy.

The provider then sends the samples to the lab for analysis. The lab can perform several types of diagnostic tests, including fluorescence in situ hybridization (FISH), standard karyotyping, and microarray or special panels. Some diagnostic test results can be ready in 72 hours, while others may take more than two weeks to complete.

Who should get genetic testing?

The decision to get prenatal genetic testing is a personal choice. Ask your healthcare provider what they recommend if you’re unsure. The results provide important information about the health of the fetus. Healthcare providers offer all pregnant families genetic screening as part of their pregnancy care.

Some of the reasons families will choose diagnostic genetic testing during pregnancy are:

• Irregular result of a genetic screening test.
• Family or personal history of genetic conditions.
• Pregnancy over age 35.
• History of miscarriage or stillbirth.

Is it necessary to get genetic testing during pregnancy?

No, it’s not necessary. It’s a personal choice based on your own beliefs or medical history. Some parents want to know if their baby will be born with a condition to plan for their care. Unfortunately, some families receive devastating results and have to decide whether to continue with the pregnancy. The choice to proceed with prenatal screening or diagnostic testing is entirely up to you and your healthcare provider.

How do they do prenatal genetic testing?

Most prenatal genetic screenings use the pregnant person’s blood. If the screening test results indicate a high risk for a congenital condition, your provider will perform more invasive tests to diagnose specific conditions. Invasive diagnostic tests include amniocentesis and CVS.

What screenings are done in the first trimester?

The first-trimester serum screening, cell free fetal DNA screening and the NT ultrasound exam all occur at 11 to 14 weeks of pregnancy. Combining information from the blood tests with the ultrasound findings indicates how high the risk is for some common chromosomal disorders, like Down syndrome.

You can have carrier screenings at any point during your pregnancy, even as early as 6 to 10 weeks gestation. These tests check for “single gene” genetic conditions that you can potentially pass to the fetus. Carrier screening won’t identify conditions due to abnormal chromosome numbers, like Down syndrome.

Cell-free fetal DNA testing (noninvasive prenatal screening or testing) checks your blood for fetal DNA. It looks for chromosomal conditions like Down syndrome, trisomy 13 and trisomy 18. Your provider can perform this screening as early as 10 weeks gestation, or later during pregnancy.

What screenings are done in the second trimester?

Second-trimester screenings occur between weeks 15 and 22 weeks of pregnancy. Second trimester blood screening tests include maternal serum alpha-fetoprotein (AFP) screen and the quad screen. It’s called a quad screen because it measures four proteins: alpha-fetoprotein (AFP), estriol, human chorionic gonadotropin (hCG) and inhibin-A. These tests can help your healthcare provider determine if the fetus is at increased risk for genetic or physical abnormalities. Fetal anatomy ultrasound is another way to screen for fetal genetic or physical abnormalities.

Can genetic screening testing be wrong for Down syndrome?

There is always a chance the prenatal screening is wrong. Your healthcare provider can provide details about the accuracy rates for any screening tests you receive during pregnancy.

Are there any risks to genetic testing during pregnancy?

Prenatal screenings don’t carry any risk. It’s a blood sample. There is a slightly higher risk if you move forward with diagnostic tests like amniocentesis or CVS. The risks for those tests include infection, bleeding or miscarriage.

When should I get my test results?

Results from genetic screening tests take several days. Results from diagnostic testing can take several days or even weeks to complete. In most cases, they’re sent to a lab for testing. Your healthcare provider will receive your test results first, then share the results with you.

What do the results of genetic screening tests mean?

Genetic screening tests measure risk only. They don’t tell you for sure if the fetus has a genetic condition. A positive result means the fetus is at a higher risk for that disorder than the general population. A negative result means the fetus is at a lower risk of having that disorder than the general population.

Your healthcare provider may suggest diagnostic testing like CVS or amniocentesis. They may refer you to a genetic counselor who specializes in high-risk pregnancies and genetic conditions. Don’t be afraid to talk to your providers about what your test results mean and the risks and benefits of diagnostic testing.

Are genetic screening tests bad for you?

No, they aren’t bad for you. Diagnostic tests like amniocentesis or CVS carry some risk. That’s why diagnostic tests are only performed when providers suspect a genetic condition and not during routine prenatal genetic screenings.

What week of pregnancy is genetic testing done?

It depends on the test. Most providers perform genetic testing after 10 weeks of pregnancy and before 22 weeks of pregnancy. At this time, neither screening nor diagnostic tests can occur prior to 10 weeks gestation.

Do genetic screening tests tell you the baby’s gender?

Cell-free DNA screening (NIPT) can give information about fetal sex in addition to the information about the risks for genetic conditions. Ultrasound may also be able to tell you about sex. Being able to determine fetal sex assignment is a bonus, not the main reason to do the test.

What questions should I ask my doctor about genetic testing?

Pregnancy screenings and diagnostic tests are a personal choice. You’ll likely have questions about what screening tests you should get or what your test results mean. Don’t be afraid to ask questions. Remember, only you and your family can decide how to handle positive results from both types of genetic tests.

Some common questions to ask are:

• What screening tests do you recommend based on my health history?
• If my screening test is positive, what are the next steps?
• Are any of the genetic tests dangerous for the fetus?
• What are the chances of false positives?

A note from Cleveland Clinic

There is no right or wrong answer when it comes to prenatal genetic testing. The decision is up to you and your family. If you’re concerned about prenatal tests or need clarification on what each test is looking for, talk to your healthcare provider. They will be able to discuss the risks and benefits of all genetic tests with you and help you make the best decision for you and your family. Remember, most babies are born healthy, but it’s important that you understand your options and what genetic tests are available to you.