Prenatal genetic screening, specifically non-invasive prenatal testing (NIPT), is an important step to check for potential complications during a pregnancy. Recent national news coverage has drawn greater attention to this issue, including a series of articles featured in The New York Times. While the focus this news coverage has drawn to this critical issue is valuable, some essential details are inaccurately represented. As a licensed and certified genetic counselor, I want to lay out the facts so patients have the clearest and most accurate picture of what they need to know about prenatal screening.
What is prenatal genetic screening?
NIPT, also known as non-invasive prenatal screening (NIPS) or cell-free DNA screening, is a method to screen if a pregnancy is at higher risk for certain genetic conditions, such as Down syndrome. Depending on the NIPT test, it may also screen for other rarer genetic conditions. NIPT is offered to pregnant individuals who are at least 10 weeks pregnant. Genetic counseling is essential for allowing individuals to make informed choices about the risks, benefits and limitations of NIPT and prenatal genetic screenings, including the choice to do no screening at all.
During NIPT screening, the pregnant individual provides a blood sample. DNA from the pregnant individual and fetal DNA are both naturally found in that blood sample. This DNA is then analyzed to calculate a risk for the pregnancy to have specific genetic conditions, primarily trisomy 21 (Down syndrome), trisomy 18 and trisomy 13. This screening can also provide information about the sex of the pregnancy.
What do NIPT results mean?
NIPT does not give a “yes” or “no” answer.
A “negative” or “low-risk” NIPT result indicates that there is a decreased risk for the pregnancy to have the genetic conditions screened for, but does not definitively rule out these or other potential genetic conditions.
A “positive” or “high-risk” NIPT result indicates there is an increased risk for the pregnancy to have a genetic condition that is screened for. It does not, however, 100% mean the pregnancy will have that genetic condition.
Across all age groups, approximately 10% of pregnant individuals will receive a false positive result. In general, a false positive NIPT result is more likely to occur in lower risk individuals (those who are at lower risk for the genetic conditions screened for). However, false positives can also occur when there are differences between the chromosomal makeup of the placenta and baby, or a previously undiagnosed chromosomal disorder in the pregnant individual. Genetic counseling offers a discussion of NIPT results, including the possibility of false positive results.
What is prenatal diagnostic testing?
Prenatal diagnostic testing is typically done through a medical procedure, either chorionic villus sampling (CVS) or amniocentesis. CVS and amniocentesis can directly analyze the pregnancy’s chromosomes and/or DNA for the genetic condition flagged by the NIPT results. For this reason, diagnostic testing should be offered to pregnant individuals to confirm any “positive” NIPT result. Diagnostic tests can give a definitive answer, but each has a risk of miscarriage (about 0.3% for CVS; about 0.2% for amniocentesis). All diagnostic testing should be offered with counseling regarding the risks, benefits and limitations so individuals can make the right choice for themselves.
Pregnant individuals who do not receive a result from their NIPT test due to a decreased amount of fetal DNA in the sample may be at increased risk to have a baby who is affected with a chromosomal disorder, and therefore need careful counseling about further testing, including the offer of diagnostic testing.
What are the limitations of NIPT?
NIPT does not screen for all potential genetic conditions, and it cannot rule out any single possibility.
NIPT can also screen for a limited number of rare genetic conditions resulting from a deletion of a portion of a chromosome, including DiGeorge syndrome, Cri-du-chat syndrome and Prader-Willi and Angelman syndromes. These conditions are extremely rare, and NIPT is less effective at screening for them. Given how rare these conditions are, there is much less data about what “positive” NIPT results look like for pregnancies affected with these rare conditions. Currently, none of the professional obstetrics or genetics organizations in the U.S. recommend routine expanded screening (such as NIPT) for any of these rarer genetic disorders.
NIPT does not estimate a risk genetic conditions that other prenatal screening tests, like maternal serum and carrier screens, can look for. Examples of this include open neural tube disorders that involve the spine, such as spina bifida.
Under certain circumstances, for pregnant individuals who have a higher body mass index, take certain medications and/or have certain health conditions, an NIPT result may not be possible due to an insufficient amount of fetal DNA in the blood sample. Certain genetic conditions can also result in an insufficient amount of fetal DNA in the blood sample. Pregnant individuals who find themselves in these categories should receive counseling from a certified genetic counselor about further testing options, including the offer of diagnostic testing.
Who should consider prenatal screening?
While NIPT is available to all pregnant individuals, those who have a higher than average chance of having a pregnancy with one of the screened conditions will be more likely to have an informative NIPT test result. This includes those who will be at least 35 years old at the time of their due date, related to their naturally increased chance of having a baby with a chromosome condition like Down syndrome.
Pregnant individuals with a naturally lower chance of having a baby with Down syndrome can also be offered NIPT (cell-free DNA screening), but are recommended to meet with a genetic counselor or other health care provider to discuss all of their testing options to make an informed choice.
Key takeaways
It is important for health care providers and patients to remember that NIPT is a screening test, and not a diagnostic test. A screening test does not tell with 100% certainty whether or not a pregnancy has a given disorder.
Certified genetic counselors can help pregnant individuals understand their NIPT results and help them navigate next steps, such as their options for diagnostic testing. If you want additional information about prenatal genetic screening or your results, you can find a certified genetic counselor at findageneticcounselor.nsgc.org.
Patrick L. Wilson, MS, LCGC is a current Board Member of the National Society of Genetic Counselors and a prenatal genetic counselor with the University of Oklahoma Health Sciences Center in Oklahoma City, Okla.
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