Nuchal Translucency. his is the maximum thickness of the subcutaneous translucent area between the skin and soft tissue overlying the fetal spine at the back of the neck (Fig. 14-2). An increased NT thickness is not a fetal abnormality but rather is a marker that confers increased risk. It is measured in the sagittal plane and is valid when the CRL value lies between 38 to 45 mm and 84 mm, with the lower limit varying according to the laboratory. Speciic criteria for NT measurement are listed in Table 10-4 (p. 186). Whenever possible, it is helpful to diferentiate increased NT from cystic hygroma, which is a venolymphatic malformation that appears as a septated hypoechoic space behind the neck, extending along the length of the back (Fig. 10-22, p. 198). Cystic hygroma confers a ivefold increased aneuploidy risk when identiied in the irst trimester (Malone, 2005a).
In addition to aneuploidy, an increased NT thickness is associated with other genetic syndromes and various birth defects, especially fetal cardiac anomalies (Simpson, 2007). And, if the NT measurement reaches 3 mm or more, the aneuploidy risk
FIGURE 14-2 Sagittal image of a normal, 12-week fetus demonstrating correct caliper placement (+) for nuchal translucency measurement. The fetal nasal bone and overlying skin are indicated. The nasal tip and the 3rd and 4th ventricles (asterisk), which are other landmarks that should be visible in the nasal bone image, are also shown. (Used with permission from Dr. Michael Zaretsky.) is unlikely to normalize using serum analyte assessment (Comstock, 2006). Because of this, if the NT measurement is at least 3 mm or exceeds the 99th percentile, the patient should receive counseling and be ofered targeted sonography with fetal echocardiography. In addition, she should be ofered cell-free DNA screening and prenatal diagnosis (American College of Obstetricians and Gynecologists, 2016c).
The NT must be imaged and measured with a high degree of precision for aneuploidy detection to be accurate. This has led to standardized training, certiication, and ongoing quality review programs. In the United States, training, credentialing, and monitoring are available through the Nuchal Translucency Quality Review program of the Perinatal Quality Foundation and through the Fetal Medicine Foundation.
Efficacy of First-Trimester Screening. Before first-trimester screening became widely adopted, four large prospective trials were conducted, together including more than 100,000 pregnancies (Reddy, 2006). When the false-positive rate was set at 5 percent, the overall rate for trisomy 21 detection was 84 percent, comparable to quadruple-marker screening (see Table 14-4). he detection rate is approximately 5 percent higher if performed at 11 weeks' compared with 13 weeks' gestation, and slightly lower-80 to 82 percent-when cases with cystic hygroma are analyzed separately (Malone, 2005a). In a recent multicenter trial, first-trimester screening detected approximately 80 percent of fetuses with trisomy 21, 80 percent with trisomy 18, and 50 percent with trisomy 13 (Norton, 2015).