Noninvasive Prenatal Testing for Fetal Aneuploidy
ABSTRACT: Noninvasive prenatal testing that uses cell free fetal DNA from the plasma of pregnant women offers tremendous potential as a screening tool for fetal aneuploidy. Cell free fetal DNA testing should be an informed patient choice after pretest counseling and should not be part of routine prenatal laboratory assessment. Cell free fetal DNA testing should not be offered to low-risk women or women with multiple gestations because it has not been sufficiently evaluated in these groups. A negative cell free fetal DNA test result does not ensure an unaffected pregnancy. A patient with a positive test result should be referred for genetic counseling and should be offered invasive prenatal diagnosis for confirmation of test results.
Noninvasive prenatal testing that uses cell free fetal DNA from the plasma of pregnant women offers tremendous potential as a screening tool for fetal aneuploidy. Circulating cell free fetal DNA, which comprises approximately 3–13% of the total cell free maternal DNA, is thought to be derived primarily from the placenta, and is cleared from the maternal blood within hours after childbirth (1). Recently, cell free fetal DNA analysis has become clinically available for women at increased risk of fetal aneuploidy.
Early attempts to detect trisomic fetuses using cell free fetal DNA required the use of multiple placental DNA or RNA markers, which made the screening test time consuming and expensive (2–4). Recently, a number of groups have validated a technology known as massively parallel genomic sequencing, which uses a highly sensitive assay to quantify millions of DNA fragments in biological samples in a span of days and has been reported to accurately detect trisomy 13, trisomy 18, and trisomy 21 (5–7) as early as the 10th week of pregnancy with results available approximately 1 week after maternal sampling. Another group has described a more targeted approach, using chromosome selective sequencing to detect trisomy 18 and trisomy 21 (8). Using archived blood samples from women who were undergoing prenatal diagnosis and were at increased risk of aneuploidy, several large-scale validation studies have demonstrated detection rates for fetal trisomy 13, trisomy 18, and trisomy 21 of greater than 98% with very low false-positive rates (less than 0.5%) (6–13). Although no prospective trials of this technology are available, cell free fetal DNA appears to be the most effective screening test for aneuploidy in high-risk women.
The American College of Obstetricians and Gynecologists has recommended that women, regardless of maternal age, be offered prenatal assessment for aneuploidy either by screening or invasive prenatal diagnosis regardless of maternal age; cell free fetal DNA is one option that can be used as a primary screening test in women at increased risk of aneuploidy (Box 1). This includes women aged 35 years or older, fetuses with ultrasonographic findings that indicate an increased risk of aneuploidy, women with a history of a child affected with a trisomy, or a parent carrying a balanced robertsonian translocation with increased risk of trisomy 13 or trisomy 21. It also can be used as a follow-up test for women with a positive first-trimester or second-trimester screening test result. Counseling regarding the limitations of cell free fetal DNA testing should include a discussion that the screening test provides information regarding only trisomy 21 and trisomy 18 and, in some laboratories, trisomy 13. It does not replace the precision obtained with diagnostic tests, such as chorionic villus sampling (CVS) or amniocentesis, and currently does not offer other genetic information. Other limitations of cell free fetal DNA include the lack of outcome data for low-risk populations; therefore, cell free fetal DNA testing is not currently recommended for low-risk women. Preliminary data available on twins demonstrate accuracy in a very small cohort, but more information is needed before use of this test can be recommended in multiple gestations (14). In a small percentage of cases, a cell free fetal DNA result will not be able to be obtained.
To offer a cell free fetal DNA test, pretest counseling regarding these limitations is recommended. The use of a cell free fetal DNA test should be an active, informed choice and not part of routine prenatal laboratory testing. The family history should be reviewed to determine if the patient should be offered other forms of screening or prenatal diagnosis for a particular disorder. A baseline ultrasound examination may be useful to confirm viability, a singleton gestation, gestational dating, as well as to rule out obvious anomalies. Referral for genetic counseling is suggested for pregnant women with positive test results. Because false-positive test results can occur, confirmation with amniocentesis or CVS is recommended. Patients also need to be aware that a negative test result does not ensure an unaffected pregnancy; false-negative test results can occur as well. In this high-risk population, a second-trimester ultrasound examination is suggested to evaluate pregnancies for structural anomalies. In patients in whom a structural fetal anomaly is identified, invasive diagnostic testing should be offered because a cell free fetal DNA test can only detect trisomy 13, trisomy 18, and trisomy 21. Maternal serum alpha-fetoprotein screening or ultrasonographic evaluation for open fetal defects should continue to be offered.
Our Thoughts and Responses:
In an opinion article written for the ACOG (American Congress of Obstetricians and Gynecologists), the ACOG considers noninvasive prenatal genetic testing to be offered to women with increased risk. For any woman regardless of risk, the ACOG discusses invasive methods.
In terms of noninvasive prenatal testing, we would like to emphasize the continuing development of this test. In the future, we hope to see noninvasive prenatal testing for aneuploidies to be offered to women regardless of risk, age or multiple gestations.
With continuing research and development, noninvasive testing will continue to give patients the knowledge they need for early detection and ability to prepare for their pregnancies.
Currently, Ravgen offers non-invasive prenatal genetic testing for aneuploidies. We encourage pregnant families to gain knowledge on their pregnancy. Please contact us with questions or for more information.