by When most people think about DNA testing, they imagine dramatic courtroom television show scenes in which the perpetrator of a crime is revealed. We now also regularly hear stories of people using DNA testing to answer questions about their ancestry, sometimes discovering blood relatives they didn’t know they had.
In the context of pregnancy, the role of DNA testing is to discover genetic diseases in the fetus. (An added bonus for parents is knowing the sex of the baby, something many are eager to know.) Unfortunately, genetic testing during pregnancy is often not as conclusive or as simple as the DNA tests used by police to determine whether someone was at the scene of an accident. crime.
A recent study shows that more comprehensive testing can be done using a maternal blood test earlier in pregnancy than the tests we currently have access to. This means that expectant parents will soon be able to know much more about their baby’s genome early on, potentially identifying many genetic diseases during the first trimester of pregnancy.
However, the results of these tests are not always clear, and patients may need help understanding what they mean and whether they should undergo additional testing.
What type of prenatal genetic testing is currently available?
Currently in the US, pregnant patients are offered two types of tests: screening tests and diagnostic tests. Screening tests are usually offered toward the end of the first trimester. One screening test is the cell-free fetal DNA test (sometimes called non-invasive prenatal testing or NIPT), which requires only a blood sample from the pregnant person. Another is the nuchal translucency test, which measures a fluid-filled space at the back of the fetus’s neck, performed using ultrasound. These tests are used to detect various chromosomal abnormalities, including Down syndrome or trisomy 21.
Because both tests are considered screening tests, they will indicate the chances that your baby has a condition, not a diagnosis. Depending on the results of those tests, you may be offered a diagnostic test to determine if the fetus actually has a genetic condition. These tests, chorionic villus sampling (CVS) and amniocentesis, are offered a little later in pregnancy: CVS between weeks 11 and 13 and amniocentesis between weeks 15 and 20. These tests examine DNA samples taken from the placenta (CVS) or the amniotic fluid surrounding the fetus (amniocentesis). Diagnostic tests can reveal a much broader range of chromosomal abnormalities by examining all of the fetus’s genes for microdeletions and duplications. Holly Mueller, a genetic counselor at the University of California, San Francisco, told HuffPost that these tests can detect about 5,000 diseases.
The advantage of diagnostic tests is that the results are very accurate and can detect a large number of conditions. The disadvantages are that they occur later in the pregnancy, which means that depending on the abortion restrictions in your state, you may have to travel to another state if you decide to terminate the pregnancy. In Florida, for example, abortion is prohibited after the 15th week of pregnancy.
There is also a small risk of miscarriage with either procedure. Mueller said the risk of miscarriage with these tests is about 1 in 450 for CVS and 1 in 900 for amniocentesis.
What tests will be available in the future?
A small study involving 36 patients was published in November in the New England Journal of Medicine. The researchers took a blood sample from each patient, as they would for a cell-free fetal DNA test, and then, using a new technology known as “trio-exome deep sequencing,” examined a large number of genetic conditions, as they would do it. with the sample obtained from an amniocentesis or a CVS test. This new test is known as the desNIPT test.
“The objective of the study is to see if we can do the same as we do with the invasive sample. [CVS or amniocentesis] … using a non-invasive test,” that is, the blood sample, Martin Larsen, one of the authors of the study, told HuffPost.
The results showed that, using this new technology, the results of desNIPT tests were as accurate as the results of an amniocentesis. Larsen, a clinical laboratory geneticist at Odense University Hospital in Denmark, said there were no false positives in any of the patients involved in the study. Several genetic diagnoses were made, including heart disease and dwarfism. Blood test results were confirmed by CVS or amniocentesis in these patients (all considered high risk). That is, both tests, the blood test and the invasive test (CVS or amniocentesis) produced the same results.
If more studies replicate this level of accuracy, desNIPT testing for many genetic conditions could become the norm in the first trimester of pregnancy.
Pregnant people could receive results earlier in pregnancy, in the first trimester (cell-free fetal DNA testing is currently offered at 10 weeks), and without the risk, albeit small, of one of the invasive tests .
Another significant advantage would be the ability to test for so many types of genetic conditions so early in pregnancy. As things stand now, Larsen explained, patients are often referred for invasive testing after suspected abnormalities are detected on an ultrasound.
“There is a group of genetic diseases where you see malformations during pregnancy, and then there is a very large group of genetic disorders where there is no indication during pregnancy that there is a problem.” In other words, there are many genetic disorders that do not show up on an ultrasound, which means the patient will not be referred for further testing.
“The potential is that this method could identify those pregnancies much earlier than what is happening now,” Larsen said.
In the report, the authors estimate a time frame of seven days for patients to receive test results.
What are the possible concerns about this new test?
Getting more results sooner sounds like a win-win situation. But there are some areas of concern.
The first is access. “One obstacle is the price of the test,” Larsen said. “It’s pretty expensive for now.” Cell-free fetal DNA testing is expensive and the new desNIPT may be even more so. He hopes more innovation will reduce the cost.
Mueller worries that in a country like the United States, which does not guarantee health care, there could be large disparities in who has access to testing. He said he already sees income-based disparities in who has access to genetic testing (people with higher incomes undergo diagnostic testing more frequently) and a new, more expensive test could further increase the disparities. .
Mueller is also concerned about accuracy, since the recent study was small and there is potential for error in the test results.
“I would really be uncomfortable, at least at this point, with patients making reproductive decisions based solely on noninvasive results,” Mueller said.
If any screening tests were positive, I would still recommend that the patient undergo follow-up diagnostic testing to be on the safe side. Mueller noted that some patients in the UK are making reproductive decisions based on a currently available non-invasive test, but this is not common practice now in the US.
Another big difference between Europe and the United States is access to abortion care. Currently, 15 U.S. states have near-total bans on abortion, meaning patients in those places need financial resources to travel for a procedure. For example, in a recent case in Texas, a woman whose fetus was diagnosed with trisomy 18 (a fatal disease) sued the state to access an abortion and ultimately left the state to terminate her pregnancy.
At the moment, Mueller said, he asks patients considering genetic testing the following questions: “Is it okay for you to potentially receive a result that will muddy the waters a little more than clear them up?” And are you willing to take additional steps after we get that result to potentially get more information, such as diagnostic testing?
“As long as they receive the appropriate advice, I am interested in patients having the tests that they consider important and relevant to them.”
