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The Truth About Prenatal Genetic Testing

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By Melissa Damrongvachiraphan, MS, LCGC

“Just tell me that my baby will be completely healthy.”

“Can’t the blood test tell me everything?”

“I want the gender test.”

I hear these kinds of statements and questions every day as a prenatal genetic counselor. The questions have increased over time with the expansion of non-invasive prenatal testing (NIPT), also known as cell-free DNA screening (cfDNA). NIPT is a screening test that analyzes DNA fragments from the pregnancy that are present in the maternal bloodstream to assess the risk for chromosome abnormalities like Down syndrome with high accuracy. The screening test can also assess the risk for less common conditions like microdeletion syndromes, but with a much lower accuracy. A recent New York Times article shed light on these issues with a dramatic claim: that the screening tests are “usually wrong.” 

The report sparked lots of discussion in the obstetric and genetics communities about the promises and pitfalls of prenatal genetic testing – from both the perspective of pregnant individuals and prenatal genetic counselors

To me, the bottom line is that both patients and providers need to work together to ensure that patients understand what genetic tests they consent to, what those tests can and cannot tell them, and what support resources are available.

Let’s set the record straight about prenatal genetic testing.

1. Not all non-invasive prenatal tests (NIPT) are equally accurate.

In general, NIPT from different labs include the core conditions that are routinely screened for in pregnancy: Down syndrome (trisomy 21), Edward syndrome (trisomy 18) and Patau syndrome (trisomy 13).  They screen these more common conditions with high accuracy and also offer the ability to learn the predicted fetal sex.

But many labs also offer screening for rarer conditions using the same blood draw—including sex chromosome aneuploidies where a person can have a missing or extra X or Y chromosome instead of the expected two X chromosomes for females or an X and a Y chromosome for males, and microdeletion/microduplication syndromes where a person has a small missing or extra piece of a chromosome. The detection rate for these additional conditions varies, but accuracy is often lower than with the core conditions.

The bottom line: It is up to your healthcare provider and you to determine which conditions you should include in NIPT.

2. Screening tests and diagnostic tests are not the same.

Screening tests like NIPT do precisely what they are supposed to do: indicate if further testing should be considered. A positive NIPT result does not mean your baby will definitely have the condition, but suggests they have an increased chance.

A screening test doesn’t definitively say whether a pregnancy has a particular condition or not; it lets you know whether there’s a need to investigate further. Screening tests by their nature have false positives, and that is why diagnostic testing is recommended as a follow-up step.

Diagnostic testing is available to anyone in pregnancy. So why not just skip screening tests and go right to the more statistically accurate diagnostic test? Because those procedures have downsides and are not necessary for every pregnancy.

Depending on the stage of pregnancy, diagnostic options called chorionic villus sampling (CVS) or amniocentesis use a small sample of cells from either the placenta or amniotic fluid to confirm or rule out a chromosome condition. They are both invasive procedures with a risk of miscarriage, ranging from 1 in 300 to 1 in 1,000 depending on the provider performing the procedure.

Most pregnant individuals and their healthcare providers choose to take a stepwise approach where they start with a screening test first, and if necessary, consider diagnostic testing after. 

The bottom line: NIPT is just a starting point and one of many tools in prenatal care.

3. NIPT is not the answer to everything.

NIPT has come a long way and continues to improve by being able to screen for more conditions with higher accuracy. But not every condition is discoverable through a blood test. Luckily, healthcare providers have other tools at their disposal.

Prenatal ultrasounds during pregnancy are one such example that allow screening for possible birth defects a blood test wouldn’t identify. And for certain inherited conditions that typically do not show anything on ultrasound, like Tay-Sachs disease, carrier screening is a way to see if parents are carriers for those inherited conditions. Carrier screening can be done during or prior to pregnancy where your DNA as a parent is analyzed to see if you are a carrier of certain autosomal recessive conditions. Like Tay-Sachs disease, autosomal recessive conditions occur when both parents must be carriers of the same condition in order to have a 25% chance of having a child with that condition. Carriers generally do not have any symptoms and there is typically no family history of the condition. Thus, carrier screening is offered to anyone who is planning for a family

Testing is not clinically available for conditions that are associated with a multifactorial inheritance, where it’s a mix of genetic, environmental, and other unknown factors that determine risks. 

The bottom line: Each testing option is a personal choice—How much information do you want to have?

4. Pre- and post-counseling allows for better understanding in options and outcomes.

The Times article got it right that more discussion and counseling both before and after testing can prevent the frustration and confusion that patients can have when they do not receive adequate information. It’s okay to ask your healthcare provider why a test is being done, how accurate it is, and if it’s required. Having the discussion beforehand can also help prepare patients for possible outcomes and next steps, as well as set expectations. Part of a healthcare provider’s role, like a genetic counselor, is to help navigate through the different options to determine which route is best for the patient and their family.

The bottom line: Talk to your healthcare provider, like a genetic counselor – or reach out to the Sarnoff Center – to learn more about the advantages and limitations of testing. 

In short, the Times article underscores the need for more truly informed consent in prenatal testing, which includes clearing up any misunderstandings of what NIPT reveals and how that information is used in follow-up care. Having a more detailed discussion with a healthcare provider, like a genetic counselor, before and after testing would address the biggest concerns outlined in the Times article. 

For questions and resources, you can also reach out to the Sarnoff Center’s genetic counselor, Melissa Damrongvachiraphan at MelissaDamron@juf.org.


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