A hypoplastic (absent or underdeveloped) nasal bone increases the likelihood that a fetus has Down syndrome ,and the likelihood that a fetus has Down syndrome increases as the length of the nasal bone (NB)  decreases [1].

Because normal nasal bone length during the second trimester appears to vary according to race and ethnic background the use of a single fixed cutoff value may be unsuitable for predicting Down syndrome in all populations [2,3,4].  Cusick and colleagues proposed a normal nasal bone length be defined as > 0.7 MoM [2]. Odibo and co-workers found a nasal bone MoM <0.75 to be the best definition of NB hypoplasia in their studies [5] . Has et al., found a nasal bone length less than 5th percentile ,  <0.72 MoM, or BPD:NBL > 12.16 could be  used in the second trimester for diagnosing fetal NB hypoplasia as a means of predicting trisomy 21 because their predictive values are similar at a fixed 5% false-positive rate [6]

The calculator below will give median   nasal bone lengths  for various populations