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Key Points
Nuchal translucency measurement in the first trimester is the most powerful marker for fetal Down syndrome.
Combination of nuchal translucency with serum markers in the first trimester detects up to 87% of cases of Down syndrome for a 5% false-positive rate.
Septated cystic hygroma, or simple nuchal translucency of 3.0 mm or greater, are indications for chorionic villus sampling (CVS) without need to await serum marker results.
First trimester absence of the nasal bones, reversal of flow in the ductus venosus, and tricuspid regurgitation may have a limited role as second-line screening tests for select high-risk patients by expert sonologists.
These second-line screening tests are unlikely to have any value for routine general population screening.
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The ideal time to screen for fetal aneuploidy is now during the first trimester of pregnancy. This evolution in screening policy is due to the significant advances that have been made in serum and sonographic markers for fetal chromosomal abnormalities over the past 20 years.
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FETAL NUCHAL TRANSLUCENCY
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The single most powerful marker available today for differentiating Down syndrome from euploid pregnancies is the first trimester sonographic measurement of the fetal nuchal translucency space. Nuchal translucency refers to the normal subcutaneous fluid-filled space between the back of the fetal neck and the overlying skin (Figure 2-1). Figure 2-2 demonstrates an increased nuchal translucency observed in a fetus subsequently shown to have Down syndrome. By adhering to a standard ultrasonographic technique, it is possible to obtain accurate measurements of this area in the vast majority of fetuses between 10 and 14 weeks' gestation. When performing nuchal translucency sonography, it is absolutely essential to ensure optimal technique, which can be attained by focusing on the following criteria (Abuhamad, 2005):
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Fetus should be imaged in the midsagittal plane, ideally with the fetal spine down.
The image should be adequately magnified so that only the fetal head, neck, and upper thorax fill the viewable area.
Fetal neck should be neutral, with care being taken to avoid measurements in the hyperflexed or hyperextended positions.
The skin at the fetal back should be clearly differentiated from the underlying amniotic membrane, either by visualizing separate echogenic lines or by noting that the skin line moves with the fetus.
Measurement calipers should be placed on the inner borders of the echolucent space, and should be perpendicular to the long axis of the fetus.
Ultrasound and transducer settings should be optimized to ensure clarity of the image and of the borders of the nuchal space in particular. This may require transvaginal sonography in certain situations.
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