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1. Fetal Medicine Foundation Criteria for Nuchal Translucency (NT)
Accurate NT measurement requires strict adherence to international quality standards to avoid misclassification.
Gestational age must be 11+0 to 13+6 weeks with a crown-rump length (CRL) between 45 mm and 84 mm.
Magnification: The fetal head and thorax must occupy at least 75% of the screen.
Mid-sagittal section: Nasal bone, nasal tip, rectangular maxilla, and transonic diencephalon must be seen.
Neutral fetal neck posture: Hyperextension falsely elevates NT by ~0.6 mm; flexion reduces it.
Distinguish the amnion from the fetal skin; calipers placed on the inner-to-inner margins perpendicular to the fetal axis.
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2. Secondary Sonographic Markers: Nasal Bone & Ductus Venosus
Evaluating secondary markers significantly enhances detection rates for Trisomy 21, 18, and 13 while reducing false-positive rates.
Nasal Bone: Present in 99% of euploid fetuses; absent or hypoplastic in ~65–70% of fetuses with Down syndrome.
Ductus Venosus Doppler: A reversed or absent 'a-wave' reflects elevated right atrial pressure and is strongly associated with chromosomal abnormalities and congenital heart disease.
Tricuspid Regurgitation: Assessed with pulsed wave Doppler; regurgitation exceeding 60 cm/s indicates elevated aneuploidy risk.
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3. Management of Increased NT (≥ 3.5 mm)
When NT is ≥ 3.5 mm (or above the 99th percentile), immediate invasive diagnostic testing (chorionic villus sampling with chromosomal microarray) and targeted early fetal echocardiography at 16 weeks are recommended.
Inform parents of elevated risk for aneuploidy, microdeletions, Noonan syndrome, and cardiac malformations.
Provide timely access to genetic counseling and non-invasive cfDNA or invasive definitive testing.