Nuchal translucency (NT) evaluation is a component of first-trimester aneuploidy screening, discussed in Chapter 14 (p. 281). It represents the maximum thickness of the subcutaneous translucent area between the skin and soft tissue overlying the fetal spine at the back of the neck. NT is measured in the sagittal plane between 11 and 14 weeks' gestation using precise criteria (Table 10-4). When the NT measurement is increased, the risk for fetal aneuploidy and various structural anomalies-in particular heart defects-is signiicantly elevated.
Assessment for selected fetal abnormalities in an at-risk pregnancy is done with irst-trimester sonography (see Table 10-2). Research in this area has focused on anatomy visible at 11 to 14 weeks' gestation, to coincide with sonography performed as part of aneuploidy screening. ith current technoloy, it is not realistic to expect that all major abnomalities detectable in the second trimester may be visualized in the irst trimester. Thus, irst-trimester scanning should not replace second-trimester anatomical evaluation (American College of Obstetricians and Gynecologists, 2016).
FIGURE 10-3 A. The measured crown-rump length is approximately 7 mm in this 6-week embryo. B. M-mode demonstrates embryonic cardiac activity and a heart rate of 124 beats per minute.
As examples, in one study of more than 40,000 pregnancies undergoing sonographic aneuploidy screening between 11 and 14 weeks, basic anatomical evaluation yielded a detection rate of approximately 40 percent for structural abnormalities (Syngelaki, 2011). Bromley and colleagues (2014) similarly found that late irst-trimester sonography identiied major abnormalities in 0.5 percent of pregnancies, representing approximately 40 percent of pregnancies with anomalies detected prenatally. Detection rates are very high for anencephaly, alobar holoprosencephaly, and ventral wall defects. But, in one analysis of more than 60,000 pregnancies with these early scans, only a third of major cardiac anomalies were identiied, and no cases of microcephaly, agenesis of the corpus callosum, cerebellar abnormalities, congenital pulmonary airway malformations, or bowel obstruction were detected (Syngelaki, 2011). In another study of low-risk or unselected pregnancies, 32 percent of anomalies were detected, whereas in pregnancies described as high-risk, anomaly detection exceeded 60 percent (Karim, 2017).
It is recommended that sonography be routinely ofered to all pregnant women between 18 and 22 weeks' gestation (American
TABLE 10-4. Guidelines for Nuchal Translucency (NT) Measurement
The margins of NT edges must be clear enough for proper caliper placement The fetus must be in the midsagittal plane
The image must be magnified so that it is filled by the fetal head, neck, and upper thorax The fetal neck must be in a neutral position, not flexed and not hyperextended
The amnion must be seen as separate from the NT line
Electronic calipers must be used to perform the measurement The + calipers must be placed on the inner borders of the nuchal space with none of the horizontal crossbar itself protruding into the space The calipers must be placed perpendicular to the long axis of the fetus The measurement must be obtained at the widest space of the NT
From the American Institute of Ultrasound in Medicine, 20 13a, with permission.
TABLE 10-5. Some Indications for Second-or ThirdTrimester Ultrasound Examination
Preterm premature rupture of membranes and/or preterm labor
Adjunct to cervical cerclage
Adjunct to amniocentesis or other procedure