A recent news article sparked a community-wide conversation about prenatal genetic testing. The article makes it seem like these tests are of little to no medical value because results can sometimes be wrong. As a clinical geneticist, I see many families who have found value in prenatal genetic tests. However, many in the general public may not understand how they should be used and many primary care providers are not trained to explain why results can be important to patients.
What Is the Purpose of Prenatal Genetic Testing?
Prenatal genetic testing can help find out if a fetus has a chromosome disorder. Chromosome disorders are often associated with multiple medical problems, such as intellectual disability and birth defects. One of the most common examples of a chromosome disorder is Down syndrome.
What Is the Risk of Having a Child With a Chromosome Disorder?
Each individual chromosome disorder is fairly rare, however, roughly 1 in 150 babies who are born has a chromosome disorder. Usually, this is a new genetic change in the baby and there is no family history of a genetic problem. Details about someone’s medical history can be used to see if there is a more specific risk for each person. For example, women who will be at least 35 years old at the time of their baby’s due date are at higher risk.
How Is Prenatal Genetic Testing Done?
Prenatal genetic testing is done by taking a sample of the mother’s blood, most commonly between 10-12 weeks of gestation. This looks at bits of the fetus’ genetic information (DNA) in the mother’s blood to try to determine if the fetus has a chromosome disorder. This process is less accurate than testing the fetus directly, however it is done this way to minimize any potential risk to the fetus.
Do Pregnant Women Have to Get Prenatal Genetic Testing Done?
Genetic testing is an optional test during pregnancy.
What Does It Mean if Prenatal Genetic Testing Is “Positive?”
This is the most important misunderstanding in the discussion around prenatal genetic testing. Prenatal genetic testing is a screening test, not a diagnostic test. A “positive” result does NOT mean that the fetus definitely has a chromosome disorder. It means that the fetus has a much higher risk of having a specific chromosome disorder. A health care provider should be able to talk about the risks, given the mother’s medical situation. Prenatal genetic counselors who are trained to give information about the test results can also be consulted.
What Should Be Done After a Positive Genetic Screening Test?
A positive screening test should be followed by a diagnostic test. This means testing the fetus directly. This can be done before birth through an invasive procedure, most often amniocentesis. Some families wait to test the baby after they are born. Testing the fetus or baby directly will tell if there is a chromosome disorder.
A family can feel much more confident that their fetus does not have a chromosome disorder if a genetic screening test is negative.
How Accurate Is Prenatal Genetic Testing for 22q11.2 Deletion Syndrome (22qds)?
It is well-known among clinical geneticists and genetic counselors that a “positive” result for the microdeletion syndromes, of which 22q is one, is more likely to be a false positive as compared to a positive result for a larger chromosome abnormality such as Down syndrome. It is important to discuss the meaning of specific results with a health care provider.
If Prenatal Genetic Testing Is Often Inaccurate, What’s the Point?
Prenatal genetic screening can be a useful tool to help the doctor and patient decide to pursue an invasive diagnostic procedure (which carries risk) or help decide if an invasive procedure is not necessary (helps to avoid risk).
It is perfectly fine if any patient, based on their own personal values and risks, decides against prenatal genetic screening. Different people, armed with the same information and in the same situation, may make different choices.
Dr. Hickey is an assistant professor of Clinical Pediatrics at Nationwide Children’s Hospital in affiliation with The Ohio State University College of Medicine.
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