At your next prenatal visit, you may be asked if you want to do testing to screen for genetic conditions. But do you really need to? What can prenatal genetic testing screen for and what do these test results mean? We’ll break it down here for you.
Overview of genetic screening tests
Non-invasive prenatal screening (NIPS) detects cell-free fetal DNA in your blood and determines whether the DNA is normal or has any missing or superfluous components. This type of screening is offered around 10 weeks of pregnancy.
Most genetic disorders are caused by microdeletions, which are small missing pieces of DNA. In others cases, they can occur when there are missing or extra copies of chromosomes, such as in Down syndrome. NIPS allows doctors to see whether this is the case early on in pregnancy, at which point diagnostic testing can be done.
How is screening different from diagnostic testing?
Prenatal screening uses a blood test that can tell you your odds of having a baby with a genetic condition. Since it only involves a blood draw in your arm, it carries with it little risk to you and your baby, but it can’t tell you whether your baby has a genetic condition.
Most genetic disorders are caused by microdeletions, which are small missing pieces of DNA.
Prenatal diagnostic testing, on the other hand, can help confirm your baby has the genetic condition that was screened for. You can choose to have diagnostic testing without first screening, but diagnostic testing is more invasive and carries with it a higher risk of miscarriage. Amniocentesis and chorionic villus sampling are the two main types of testing.
What genetic disorders can you screen for?
Although prenatal genetic screening is continuing to expand, it can’t show you every genetic disorder. With NIPS, you can mainly screen for:
- Trisomy 21, or Down syndrome: Caused by an extra chromosome 21 and affects 1 in 700 babies. Babies have distinct physical features, intellectual disabilities, and low muscle tone.
- Trisomy 18, or Edwards syndrome: Caused by an extra chromosome 18 and affects 1 in 5,000 babies. Babies have a small head, jaw, and mouth along with cleft palate and intellectual disabilities.
- Trisomy 13, or Patau syndrome: Caused by an extra chromosome 13 and affects 1 in 16,000 babies. Babies can have extra fingers or toes, and heart and brain birth defects.
- Sex chromosome abnormalities: An X or Y chromosome (the sex chromosomes) is missing or has extra copies. Some common sex chromosome disorders are Turner syndrome, Triple X syndrome, Klinefelter syndrome, and Jacob’s syndrome.
NIPS can screen for other more rare disorders caused by microdeletions including:
- DiGeorge syndrome: Also known as 22q11.2 deletion syndrome. It’s caused by a missing piece from chromosome 22, and babies can have heart defects, developmental delays, and cleft palate. It affects 1 in 3,000 to 6,000 babies.
- Monosomy 1p36: Caused by a missing piece of chromosome 1. Babies can have intellectual disabilities, heart conditions, and seizures. Affects 1 in 5,000 to 10,000 babies.
- Angelman syndrome: Caused by chromosome 15 missing a piece and affects 1 in 12,000 to 20,000 babies. Babies can have developmental and speech delays and concerns with movement and balance.
- Cri du chat syndrome: Also called 5p-syndrome, there is a missing piece from chromosome 5. It affects 1 in 15,000 to 50,000 babies. Signs include a high-pitched cry in newborns, low birth weight, a smaller than average head size, and physical milestone delays.
- Prader-Willi syndrome: Caused by a missing piece from chromosome 15 and can affect 1 in 10,000 to 30,000 babies. There can be developmental delays, low muscle tone, and distinct facial features.
There are hundreds of microdeletion syndromes, but most tests only look for a few of these. This means if your genetic screening came back negative, it doesn’t mean your baby doesn’t have a genetic disorder. But it does typically mean you don’t need to do any further testing.
If you get a positive screening, talk with your doctor about what your next steps should be. You may need additional testing to confirm your results.
According to Mayo Clinic, accuracy rates vary depending on the screening test. For instance, screening can accurately identify Down syndrome about 85 percent of the time.
However, NIPS isn’t approved by the Food and Drug Administration (FDA). In a statement, the FDA said manufacturer claims of accuracy don’t necessarily follow “scientific evidence.” The American College of Obstetricians and Gynecologists (ACOG) said in another statement that it doesn’t recommend NIPS when screening for microdeletions.
What if you get a positive screen?
A positive result means you have an increased chance of having a baby with that particular condition you screened for. Each report will have a statistic known as a positive predictive value and those percentages vary depending on your age and how far along in your pregnancy you are. If you get a positive screening, talk with your doctor about what your next steps should be. You may need additional testing to confirm your results. It can be difficult to hear this information, but remember that screening is different from testing and doesn’t necessarily mean there’s a concern with your baby. In fact, according to investigations by The New York Times, the vast majority of women who have positive screenings don’t actually have a baby with a genetic condition, because false positives may occur.
With a false positive, you get a positive result from the screen but your baby doesn’t have the condition (revealed definitively by further testing). While NIPS is reliable in determining a baby’s chance of Down syndrome, it’s less accurate in screening for other genetic disorders, according to Labcorp.
If further testing does reveal you have a baby with a genetic condition, there is time to learn about the diagnosis and make a plan for the challenges that might arise.
Should you get NIPS?
There’s a lot to consider when deciding whether or not to have prenatal screening.
First, it can be expensive. You’ll need to check with your insurance company, since not all of them cover prenatal screening. Out-of-pocket costs can vary from several hundred to thousands of dollars.
According to the American College of Obstetricians and Gynecologists, screening is offered to all pregnant people regardless of whether they are at higher risk for a baby with a genetic condition or not. It used to be that only women over age 35 were screened for chromosome disorders, because there is a higher chance of having a baby with a chromosome disorder in this age group. Even if your doctor recommends NIPS, it doesn’t mean you need to have it done.
For some women with a family history of genetic conditions though, this screening may be important.
You’ll have to consider that screening for genetic disorders can cause a lot of stress and worry. They also may not be completely accurate, with either a false positive or negative. If you get a positive screen, a diagnostic test usually isn’t done until the second trimester, so you’ll have to wait a few weeks, which can prolong the anxiety. For some women with a family history of genetic conditions though, this screening may be important.
If you’re considering screening for genetic conditions, it’s a good idea to talk with a genetic counselor who can advise you on personal risk factors and help you decide the best plan for you and your family.