ABSTRACT
The association of nasal bone hypoplasia and trisomy 21 is well established. There are ultrasound studies in low- and high-risk populations, as well as histopathological studies in fetuses with Down syndrome, which have consistently confirmed this association. It is established that 50%–60% of fetuses with Down syndrome will have absent/hypoplastic nasal bones at 11–14 weeks on ultrasound scanning and also on autopsy studies. Advanced maternal age, increased nuchal translucency, abnormal serum biochemistry, and abnormalities or soft markers seen on ultrasound scans are the factors to be considered in determining the a priori risk and would define a “high-risk” population. Absence of nasal bones would significantly increase the chance of underlying trisomy 21, and an invasive test should be considered. On the other hand, prevalence of trisomy 21 in the low-risk population is low. Failure to visualize the fetal nasal bones could easily be a variation of normal.
