Disclaimer :: This helpful explanation of new technologies in medicine pertinent to pregnant mamas is sponsored and written by none other than by Dr. Catherine Bevan, a local Tarrant County OB/GYN.
In today’s sometimes frustrating health care climate, it’s easy to overlook the breakthroughs and scientific advancements that are occurring every day at universities and corporate laboratories. The utilization of cell-free fetal DNA is a perfect example of one such advancement. It is fascinating to me that there is DNA from the fetus just hanging out circulating in the mother’s bloodstream. The fact that we now have the technology to isolate this fetal DNA from a blood sample from mom and reliably test for common genetic issues is even more fascinating. The implications of this are very personal to me, not just as an obstetrician, but as a mother who has utilized this testing myself.
Testing for the most common chromosomal abnormalities (an extra copy of chromosome 13, 18, 21, and an extra or absent sex chromosome X or Y) is how cell-free fetal DNA (cfDNA) is currently utilized in the laboratory setting. These tests are complicated, relatively time-consuming, and expensive. Part of this is due to that fact that this type of testing has only been offered for a relatively short period of time. As more and more laboratories develop cfDNA tests, the costs will hopefully go down. Due to its novelty, the best practices for utilizing cfDNA prenatal testing have yet to be established. The test can be offered as early as nine weeks gestation and has the added bonus of determining the sex of the fetus, if desired.
Screening Test vs Diagnostic Test
Prenatal screening using cfDNA is a screening test, and it is not used to diagnose chromosomal abnormalities such as Down’s syndrome (Trisomy 21). Like the conventional prenatal screening methods, cfDNA testing is designed to catch all fetuses who may have a certain condition. This invariably leads to false positive results (i.e. where the test is positive, but the fetus is not in actuality affected by the condition). In fact, in a general obstetric population (low-risk), only about one in three women whose fetus test positive for Down’s syndrome actually have the condition. Diagnostic testing including chorionic villous sampling or amniocentesis is necessary to diagnose the condition with certainty. A major benefit to cfDNA testing is that if the testing comes back negative then there is very little chance that the fetus is affected (a low false negative rate). There are a lot of statistics involved that is beyond the scope of this post, but the takeaway is that cfDNA is a reliable screening test, but further diagnostic testing is necessary to diagnose a chromosomal abnormality with certainty.
Limitations of cfDNA
In addition to the occasional false-positive results, the results of cfDNA testing may come back as “no call,” meaning that the results are indeterminate or uninterpretable. This can be due to several factors, but is most commonly a consequence of a low fetal fraction (not enough fetal DNA in mom’s blood stream compared to maternal DNA). Additionally, cfDNA testing only assess risk for chromosomal abnormalities, not other fetal anomalies such as neural tube defects like spina bifida.
Who Should Be Offered cfDNA Testing
This is a complicated issue for which there is currently no clear consensus. In high risk populations, such as women over age 35, cfDNA testing is routinely offered. In low-risk patients, it depends on the practice, patient desires, cost considerations, etc. For many patients, a positive or negative test result would not affect their pregnancy decisions. In these cases, patients may decide to decline all screening for chromosomal abnormalities entirely. Like all healthcare decisions, an open discussion with your provider is paramount to achieving the best outcomes.
Dr. Catherine Bevan is a board certified obstetrician and gynecologist, who practices in Tarrant County, with offices in Fort Worth and Willow Park. She is a Fort Worth native and received her Bachelor of Arts degree from Washington University in St. Louis where she graduated cum laude, Phi Beta Kappa. She then returned to Texas to complete her medical degree at UT Southwestern as well as her residency training at Parkland Memorial Hospital. She also spent an additional year as an assistant professor at UT Southwestern, specializing in gynecologic surgery and emergency women’s care. Providing excellent service to her patients is a passion for her as she guides them to the best possible healthcare outcomes.
You can read more from Dr. Bevan on the Fort Worth Moms Blog archives.
You can contact Dr. Bevan at:
- 1250 8th Avenue, Ste 320 // Fort Worth, TX 76104 // (P) 817-924-2111 // (F) 817-564-3980
- 134 El Chico Trail, Suite 103 // Willow Park, TX 76087 // (P) 817-441-1644 // (F) 817-441-1626