
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.