by The Society for Maternal-Fetal Medicine (SMFM) with the assistance of Mary E Norton, MD; Joseph R Biggio, MD; Jeffrey A Kuller, MD; and Sean C Blackwell, MD
The introduction of cell-free DNA (cfDNA) screening for aneuploidy into obstetric practice in 2011 revolutionized prenatal testing. The American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) both recommend that all women should be offered the option of aneuploidy screening or diagnostic testing for fetal genetic disorders.
The most recent guidance addressing this issue suggests that traditional screening with serum markers and nuchal translucency (NT) measurement remains the most appropriate option for low-risk patients, while in women at higher risk for common aneuploidies, cfDNA screening may be more accurate. In addition, SMFM has stated that due to the ethics of patient autonomy, after appropriate counseling regarding the benefits and limitations of cfDNA screening, this option should be available to women who request testing beyond what is currently recommended by professional societies.
The number of different screening and testing options raises questions about how to incorporate cfDNA screening into traditional approaches to screening. Here we review the current data on the role of ultrasound in women who have undergone or are considering cfDNA screening.
Q: What is the role of NT measurement in women who plan to have or have already had cfDNA screening and received a negative or low-risk result?
The current ACOG and SMFM guidance states that NT measurement for aneuploidy risk is not necessary at the time of cfDNA screening in the first trimester. However, ultrasound examination is useful to confirm viability, to confirm the number of fetuses and the presence of an empty gestational sac, to assign gestational age, and to identify some major fetal anomalies for patients who may choose to have cfDNA screening. Those who choose serum integrated screening may be offered first-trimester ultrasound for gestational dating even if NT measurement is unavailable or cannot be obtained. If an enlarged NT, an obvious anomaly, or a cystic hygroma is identified on ultrasound, the woman should be offered genetic counseling and diagnostic testing for aneuploidy as well as follow-up ultrasound for fetal structural abnormalities.
Therefore, in women who are considering having cfDNA screening, first-trimester NT assessment may help them to choose between screening and diagnostic testing. In women with a negative cfDNA screen, first-trimester NT measurement is of limited additional benefit as a screening test for aneuploidy or structural abnormalities. This is due to the fact that the detection rate for trisomies 21, 18, and 13 by cell-free fetal DNA is sufficiently high that if the result is negative/low-risk, the NT measurement provides little additional information. Detection of some anomalies is possible as early as 11–14 weeks’ gestation; however, ultrasound to screen for major structural abnormalities in the first trimester should not replace screening of fetal anatomy in the second trimester.
Q: Should the presence of soft markers of aneuploidy be reported in women who have already had cfDNA screening?
The concept of soft markers was introduced in an era predating methods of screening for Down syndrome other than maternal age, when the detection rate for Down syndrome was only 20%–30%. This approach was promoted as a means to detect aneuploidy in otherwise low-risk women who had no other screening options. Since the sensitivity of cfDNA screening for Down syndrome approaches 99%, the residual risk for Down syndrome is exceedingly low in patients who have had a negative cfDNA screen.
Recommending diagnostic testing in women when a soft marker is identified would, however, result in a substantial increase in the number of diagnostic tests. Therefore, we recommend that diagnostic testing not be recommended to patients solely for an isolated soft marker in the setting of a negative cfDNA screen. Because the residual risk for Down syndrome in a woman with a negative cfDNA screen is so low, when an isolated soft marker is noted on second-trimester ultrasound examination, the sonographer may choose to state that it is a normal variant or likely of no clinical significance.
When more than one marker is found, the likelihood of aneuploidy is higher than in the presence of an isolated marker, but the actual magnitude of the increase depends on the markers. Due to these complexities and the limitations of prenatal ultrasound in the setting of a negative cfDNA screen with multiple soft markers, genetic counseling should include consideration for diagnostic testing. Prenatal risk assessment for aneuploidy and/or chromosomal abnormalities based on soft markers should be limited to individuals and centers with training and/or experience in prenatal diagnosis
Some soft markers (eg, choroid plexus cysts and echogenic intracardiac foci) have minimal clinical significance in the absence of a higher pretest risk of fetal aneuploidy. However, other sonographic soft markers, such as mild pelviectasis and echogenic bowel, may indicate a fetal abnormality other than aneuploidy. Even if cfDNA screening has been performed, such cases require additional prenatal or postnatal evaluation.
Q: When soft markers of aneuploidy are detected in an otherwise low-risk woman, should cfDNA screening be offered?
In general, isolated soft markers have limited utility in detection of aneuploidy in low-risk patients. In women who have already had traditional aneuploidy screening with normal results, risk of trisomy 21 typically remains low even in the presence of an isolated soft marker given the low a priori risk and the relatively low positive likelihood ratios for the soft markers.
Although the addition of negative cfDNA screening could potentially alleviate patient anxiety, it has been reported that even with normal diagnostic testing, presence of soft markers is still anxiety-provoking, and many patients are not completely reassured by normal results on diagnostic testing. Presumably, negative results on cfDNA screening would likewise not completely alleviate anxiety. Therefore, providers should carefully consider a consistent approach to such findings. For a woman who has an identified isolated soft marker on a second-trimester ultrasound in the setting of a negative serum screen, a reasonable approach is to consider the presence of the isolated soft marker to be a “normal variant.” In an effort to achieve further reassurance (without the risks of diagnostic testing), cfDNA screening may be made available to these patients, however, they should be counseled that their risk of aneuploidy is low based on their initial screening result and the amount of additional risk reduction with a negative cfDNA screening result is unclear.
Q: Should cfDNA screening be offered to women when structural fetal anomalies are detected?
The presence of fetal structural abnormalities significantly increases the risk that a fetal chromosomal abnormality or a copy number variant detectable by microarray is present. While those aneuploidies detectable by cfDNA screening make up a significant proportion of such abnormalities, many structurally abnormal fetuses have chromosomal abnormalities that are not detectable by cfDNA screening.
Women who decline diagnostic testing may request cfDNA screening as an alternative. If this approach is chosen, they should be counseled that there is a substantial risk that a chromosomal abnormality other than trisomy 21, 18, and 13 is present and will not be detected by cfDNA screening.
While some expanded cfDNA screening panels have been reported to detect a select few targeted microdeletions, and one panel offers evaluation of all chromosomes at a resolution of 7 megabases (similar to that of conventional karyotype), the detection and false-positive rates of such panels have not been studied in prospective clinical trials. In addition, such panels are able to detect only a very small percentage of the total number of copy number variants that can be identified with chromosomal microarray.
Pregnancy management should not be altered solely based on the results of cfDNA screening, as false-positive and false-negative results are possible. For women who decline diagnostic testing in the setting of fetal structural abnormalities, pregnancy management should depend on the entire clinical scenario including the specific abnormalities present, gestational age, and preferences of the woman, as well as the results of the cfDNA screen.