Prenatal Genetic Testing

Prenatal genetic testing helps detect certain genetic disorders or birth defects that your baby may be born with. Changes to a person’s genes or chromosomes cause genetic disorders. Healthcare providers can diagnose some of these conditions before birth.

The choice to do prenatal screening or diagnostic testing is entirely up to you. Your pregnancy care provider can tell you what they recommend based on your family history, age during pregnancy and other factors.

Types of tests

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

• Screening tests: Estimate the chances that your baby will be born with a condition
• Diagnostic tests: Confirm if your baby will be born with a condition

There are many different types of tests. They’re usually done at different times during pregnancy. The tests may involve giving a blood sample, having an ultrasound or taking a small sample of tissue or fluid.

Screening tests are usually done first and carry little risk. Diagnostic tests are more accurate but carry risks.

What conditions can be detected through genetic testing during pregnancy?

They can detect conditions caused by:

• Extra or missing chromosomes, like Down syndrome, trisomy 18 and trisomy 13
• A change in one gene, like cystic fibrosis, spinal muscular atrophy (SMA) or sickle cell disease
• Missing pieces of chromosomes, like DiGeorge syndrome

Prenatal screenings

Screenings don’t diagnose conditions. The results only show the chances of your baby having a condition. Your healthcare provider can explain your results and the next steps. In some cases, they may recommend diagnostic testing.

Some examples of prenatal genetic screening tests are:

Carrier screening

Carrier screening checks for small changes in genes that increase the risk of single-gene conditions. It uses a blood sample. If your test shows you’re a carrier, your partner should also be tested. If both of you are carriers for the same condition, your baby could inherit the disease. You can get this screening during pregnancy or before you become pregnant.

Screening for an abnormal chromosome number

Chromosomes are inherited in pairs — one half of the pair from each parent. Sometimes, errors happen in the development process. This leads to missing or extra parts of the chromosome pair. Examples include Down syndrome (an extra chromosome 21) and Turner syndrome (a missing X chromosome). You’ll need this test each time you’re pregnant. It’s a blood test.

Specific types of tests include:

• Cell-free fetal DNA screening: This is also called 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 sample. You can’t have this test until 10 weeks of pregnancy.
• Serum screening: This 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 include sequential screening, quad screening and first trimester serum screening. 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. Ultrasound and blood tests can help determine the fetus’s risk for physical abnormalities.

• Nuchal translucency: This uses ultrasound to measure the thickness of the back of the fetus’s neck. A thick nuchal translucency gives clues about the risk for abnormal chromosome number and for irregular heart formation. Providers perform this ultrasound at 11 to 14 weeks.
• AFP screening: Your provider draws some of your blood to measure the level of AFP (alpha-fetoprotein) in your blood. High levels may mean there are physical problems with the fetus’s abdomen, face or spine. This screening occurs between 15 and 22 weeks.
• Quad screen: This shows the levels of four substances in your blood. It determines the risk of the fetus having chromosomal abnormalities and neural tube defects. This test is also called a multiple marker screen. It happens between 15 and 22 weeks.
• Anatomy scan: This uses ultrasound to look at the fetus’s brain, skeleton, heart, kidneys, abdomen, face, arms and legs. Healthcare providers usually do this ultrasound at 18 to 20 weeks of pregnancy.

Remember, a prenatal screening tells you a condition might be present. It doesn’t diagnose a condition.

Prenatal diagnostic tests

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 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. 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.

Your provider sends the samples to a lab for analysis. The lab can perform several types of diagnostic tests on the sample. Some results can be ready in 72 hours, while others may take more than two weeks to complete.

Who should get genetic testing?

Having genetic testing during pregnancy is a personal choice. Healthcare providers offer all pregnant families genetic screening as part of their pregnancy care. Ask your provider what they recommend if you’re unsure.

Some of the reasons families choose diagnostic genetic testing are:

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

How do the tests work?

Most prenatal genetic screenings use the pregnant woman’s blood. If the screening test results show a high risk for a congenital condition, your provider will recommend diagnostic tests like amniocentesis and CVS. These involve testing a sample of amniotic fluid or placental tissue.

Are there risks to genetic testing during pregnancy?

Prenatal screenings are very low risk because they’re blood tests.

There are risks to diagnostic tests like amniocentesis or CVS. The risks for those tests include infection, bleeding or miscarriage. Talk to your healthcare provider to make sure you understand the risks.

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.

Your healthcare provider will go over the test results with you. If you receive the results in your electronic medical records before you talk to your provider, try not to panic about the results. Your provider is the best person to interpret your test results.

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 if your screening results are abnormal. 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.

How accurate is genetic testing during pregnancy?

Screening tests are less accurate. The accuracy depends on the type of test, though. For example, non-invasive prenatal testing (NIPT) is about 99% accurate in detecting Down syndrome.

Diagnostic tests like amniocentesis or CVS are much more accurate, usually over 99%.

You should discuss your test results and how accurate they are with your healthcare team.

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

Cell-free DNA screening (NIPT) can give information about sex. Ultrasound may also be able to tell you about sex. Learning the sex is a bonus. It's not the main reason to do genetic testing.

What questions should I ask my doctor about genetic testing?

You’ll likely have questions about which 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 genetic tests.

Some common questions to ask are:

• Which screening tests do you recommend based on my health history?
• If my screening test is positive, what are the next steps?
• What are the chances of false positives?
• What are the risks of each type of test?