For three decades, investigators have recognized that the sonographic detection of aneuploidy, particularly Down syndrome, may be improved by minor markers that are collectively referred to as "sot signs." Minor markers are normal variants rather than fetal abnormalities, and in the absence of aneuploidy or an associated abnormality, they do not significantly afect prognosis. They are present in at least 10 percent of unafected pregnancies (Bromley, 2002; Nyberg, 2003). Examples of these sonographic findings are listed in Table 14-8 and depicted in Figure 14-3. Findings are generally useful from 15 to 20 or 22 weeks' gestation. Six of these markers have been the focus of sonographic studies, in which likelihood ratios have been derived that allow a numerical aneuploidy risk to be calculated (Table 14-9). The risk rises steeply with the number of markers identiied. Alternatively, absence of a minor marker has been used to reduce the calculated risk (Agathokleous, 2013). This should be done systematically, following a protocol that specifies the markers included in a model, the definition for what constitutes a inding, and positive-and negative-likelihood ratios (Reddy, 2014).
he nuchal skinod is measured in the trans cerebellar view of the fetal head, from the outer edge of the skull to the outer border of the skin (see Fig. 14-3A). A measurement �6 mm is typically considered abnormal (Benacerraf, 1985). This inding is present in approximately 1 per 200 pregnancies and confers a more than tenfold risk for Down syndrome (Bromley, 2002; Nyberg, 2001; Smith-Bindman, 2001).
An echogenic intracardiac ocus is a focal papillary muscle calcification that is neither a structural nor functional cardiac abnormality. It is usually left-sided (see Fig. 14-3B). Such a focus is present in approximately 4 percent of fetuses, but it
TABLE 14-8. Second-Trimester Sonographic Markers or "Sot Signs" Associated with Fetal Trisomy 21 a
Clinodactyly (hypoplasia of the 5th digit middle phalanx) Echogenic bowel
OUsted alphabetically.
may be found in up to 30 percent of Asian individuals (Shipp, 2000). As an isolated finding, this approximately doubles the risk for fetal Down syndrome (see Table 14-9). Bilateral echogenic foci are associated with trisomy 13 (Nyberg, 2001).
Y1ild renal pelvis dilatation is usually transient or physiological and does not represent an underlying abnormality (Chap. 10, p. 208). The renal pelves are measured in a transverse image of the kidneys, anterior-to-posterior, with calipers placed aHigher in Asian individuals.
Data from Bromley, 2002; Nyberg, 2001; Smith-Bindman, 2001l.
at the inner borders of the fluid collection (see Fig. 14-3C). A measurement �4 mm is found in about 2 percent of fetuses and approximately doubles the risk for Down syndrome. he degree of pelvic dilatation beyond 4 mm correlates with the likelihood of an underlying renal abnormality, and additional evaluation is generally performed at approximately 32 weeks.
Echogenic etal bowel is deined as bowel that appears as bright as fetal bone (see Fig. 14-3D). It is identiied in approximately
FIGURE 14-3 Minor sonographic markers that are associated with increased risk for fetal Down syndrome. A. Nuchal skinfold thickening (bracket). B. Echogenic intracardiac focus (arrow). c. Mild renal pelvis dilatation (pyelectasis) (arrows). D. Echogenic bowel (arrow).