SMFM Consult: Ultrasound in the cfDNA era
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.