Prenatal Genetic Testing: Which Tests Do You Need?
Trying to conceive or newly pregnant? You’ve likely already started dreaming up baby names and imagining who your little bundle will resemble. But there’s another big question worth considering: Should you get genetic testing?
What is prenatal genetic testing?
Prenatal genetic testing helps determine if your baby-to-be has a problem with its genes or chromosomes. If you think all the way back to high school bio class, you might remember that we have 23 pairs (46 total) chromosomes that house our genes. Our genes, of course, give us traits like our blood type or eye color, and they also hold information about our health. You get one chromosome from each parent (or donor, if using a donor egg and/or sperm) to make those pairs. However, in rare cases errors occur, resulting in an extra chromosome (known as trisomy) or a missing chromosome (monosomy). Having an extra or missing chromosome causes conditions such as Down Syndrome or Trisomy 18 (Edward’s Syndrome). And then there are other genetic mutations that can result in serious diseases such as Tay-Sachs or Cystic Fibrosis.
Prenatal genetic testing can help spot some of those disorders early in pregnancy. There are two different categories of prenatal genetic testing: screening tests, which tell you the likelihood that your offspring will have a disorder, and diagnostic tests, which can diagnose those disorders with more than 99.9% accuracy.
Prenatal Genetic Screening Tests
Prenatal screening tests assess risk. It’s super important to know that screening tests do not prove conclusively that a disease is present or not. Screening tests simply identify how likely it is that a genetic disorder is present. For example, a pregnant person might be told that based on genetic screening, their baby has a 1 in 400 chance of having Down Syndrome. That doesn’t mean your baby definitely will or will not have Down syndrome. If a screening test indicates a high probability of a genetic disorder, your practitioner will likely recommend moving forward with a diagnostic test.
Types of Prenatal Screening Tests
The following are commonly used prenatal genetic screening tests:
Carrier Screening Tests
Some people carry a gene for a disease without actually having the disease. Often that’s because it’s a recessive disorder, which requires two genes (one inherited from each party) to show up. If you only have one of those genes, you’re a carrier. If your partner is also a carrier, that means your child has a 25% chance of having that disorder and a 50% chance of being a carrier themselves).
A blood or saliva test can determine if you or your partner are carriers of genetic diseases such as Tay-Sachs, Fragile X syndrome, Sickle Cell Disease, Spinal Muscular Atrophy, or Cystic Fibrosis. If you belong to a high-risk group, scheduling a carrier screening before you become pregnant gives you more options. For instance, with IVF, embryos can be tested for genetic abnormalities before they are implanted.
Usually, the partner that’s higher risk gets tested first. If the first partner tests negative (meaning they don’t have the gene), no additional tests are needed. If you test positive (meaning you do have the gene), your partner should test too. Carrier screening results can be confusing, but a genetic counselor can help you interpret your test results.
First Trimester Prenatal Screening Tests
Conducted between 10 weeks and 13 weeks, first trimester screenings include a maternal blood test and Nuchal Translucency Screening—an ultrasound that measures the thickness of the nuchal fold, a space at the back of the baby’s neck. A thicker than normal nuchal fold is a marker for several abnormalities, including Down Syndrome.
Second Trimester Prenatal Screening Tests
Conducted between 15 weeks and 22 weeks, second trimester screening includes a blood test called the Quad Screen (or quadruple marker). It looks for proteins in the pregnant person’s blood that may indicate Down syndrome, Trisomy 18, or neural tube defects such as spina bifida. Also during the second trimester, a thorough ultrasound around 20 weeks (aka the anatomy scan) can show evidence of other birth defects such as cleft palate or heart problems.
Cell-Free Fetal DNA Testing
Sometimes referred to as non-invasive prenatal testing (NIPT), cell-free DNA testing can be conducted at week 10 with a simple blood test. This screening test isolates the small particles of fetal DNA that are found in the mother’s blood and is able to predict more accurately than other screening tests if your baby will have Down syndrome or other chromosomal disorders. However, it does not check for issues with the brain or spine. As a bonus, this test can reveal the baby’s gender. Unfortunately, some insurance plans don’t cover this test if you don’t have certain risk factors.
Do I need prenatal genetic screening?
Some doctors recommend screening for all their patients. Others focus on patients that fall into higher risk categories, including:
- Either parent being older (a pregnant person over 35 is typical considered “advanced maternal age”)
- Having another baby with a birth defect
- Previous pregnancy complications, such as preeclampsia, gestational diabetes, or premature birth
- More than one past miscarriage
- A genetic disorder runs in your family or your partner’s family
- Being a member of an ethnic group that has a high incidence of a certain disease
- Having a particular medical condition such as diabetes, high blood pressure or an auto-immune disease
Prenatal Genetic Diagnostic Tests
Prenatal diagnostic tests give you a more definitive answer about your baby and they’re typically performed after you’ve done a screening test. Diagnostic tests look at cells from the placenta or fetus. Because they’re more invasive, they carry some risk. That’s why screening tests are always performed first—only higher-risk pregnancies move forward with diagnostic tests.
Types of Prenatal Genetic Diagnostic Tests
There are two main types of prenatal genetic diagnostic tests: amniocentesis and chorionic villus sampling (CVS):
- Chorionic villus sampling (CVS): Typically administered between 10 and 13 weeks of pregnancy, a thin tube is inserted into the cervix to collect a small tissue sample from the placenta.
- Amniocentesis: A needle is inserted through the abdomen to collect a sample of amniotic fluid for testing. Amnios are typically performed between 15 and 20 weeks of pregnancy but can be given right up until birth.
During both of these tests, ultrasound guidance allows your practitioner to avoid contact with your baby. But the tests still carry a small risk. Sometimes miscarriage occurs after a diagnostic test—however, the risk is less than 1% risk. Research suggests that the risk is even much lower than the 1% risk that’s cited. This includes two recent studies—2021 Slovenian research and a 2019 study published in the journal Ultrasound in Obstetrics & Gynecology—that found the risk of both procedures was negligible when compared to a control group with a similar profile. Still, this risk, no matter how tiny, is why these tests are typically only used to confirm a positive genetic screening test result.
If your provider recommends diagnostic testing, you will likely need to choose between CVS and amniocentesis. Like everything, there are pros and cons for each. The miscarriage risk for amniocentesis is slightly lower than CVS, but CVS can be performed earlier in the pregnancy. You can always discuss your options with your OB/GYN or a genetic counselor if you have any questions about the right choice for you.
Do I need prenatal genetic testing?
It’s up to you to decide what feels right! There are several reasons that expecting parents say yes to prenatal genetic testing. Some may want time to plan for medical or hospice care for a baby with a serious diagnosis. Others may want the option of terminating a pregnancy if the fetus has a severe genetic abnormality, particularly if it’s not compatible with life. Or, you might simply want to give yourself time to adjust to and learn about the diagnosis before sharing with family and friends. And, of course, other folks may not want this information in advance at all. In the end, it’s a very personal decision.
How long does genetic screening take?
The in-office procedures are not lengthy, but it can take a week to 10 days to get results back for blood tests, CVS and amniocentesis, and even longer to test for certain rare disorders. Waiting for your genetic screening results can be stressful, so make sure to treat yourself extra kindly during this time (and maybe even plan something fun to take your mind off of your test results).
Final Thoughts on Genetic Testing
During pregnancy, most expecting parents will go through with screening tests to assess their baby’s risks and fewer will need to do diagnostic tests. But ultimately, prenatal genetic testing is your choice. Talking in depth with your doctor and—depending on your risk—perhaps consulting a genetic counselor can help you in the right direction and chart the course that’s right for you.
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REFERENCES
- Understanding Genetics: A New York, Mid-Atlantic Guide for Patients and Health Professionals, Genetic Alliance: The New York-Mid-Atlantic Consortium for Genetic and Newborn Screening Services, July 2009
- National Library of Medicine: Tay-Sachs Disease
- National Library of Medicine: Cystic Fibrosis
- National Library of Medicine: Trisomy 18
- National Library of Medicine: Down Syndrome
- The American College of Obstetricians and Gynecologists: Prenatal Genetic Screening Tests
- The American College of Obstetricians and Gynecologists: Prenatal Genetic Diagnostic Tests
- Risk of Miscarriage Following Amniocentesis or Chorionic Villus Sampling: Systematic Review of Literature and Updated Meta-Analysis, Ultrasound in Obstetrics & Gynecology, May 2019
- Pregnancy Loss After Amniocentesis and Chorionic Villus Sampling: Cohort Study, Zdravstveno Varstvo, March 2021
- The American College of Obstetricians and Gynecologists: Carrier Screening
- Your Pregnancy and Childbirth Month to Month, The American College of Obstetricians and Gynecologists
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