Which ultrasound finding is most sensitive for identifying Trisomy 21?

The absence or hypoplasia of the nasal bone is a particularly sensitive ultrasound marker for Trisomy 21, also known as Down syndrome. In the context of prenatal screening, the presence of a normal nasal bone is seen in the majority of fetuses without chromosomal abnormalities. Conversely, a significant proportion of fetuses with Trisomy 21 demonstrate either absent or hypoplastic nasal bones. This finding becomes even more meaningful when combined with other markers of Down syndrome in biomarkers screening, increasing its predictive value.

Short femur, echogenic bowel, and microcephaly are associated with various conditions but do not have the same level of sensitivity specifically for Trisomy 21. While a short femur may suggest abnormal fetal growth patterns and echogenic bowel can indicate several gastrointestinal or chromosomal anomalies, neither provides the focused association with Trisomy 21 that the nasal bone finding does. Microcephaly can be observed in numerous genetic and environmental conditions as well and does not specifically correlate with Trisomy 21. Thus, the absence or hypoplasia of the nasal bone provides the most direct link to identifying this particular chromosomal anomaly.