Likelihood ratios for fetal trisomy 21 based on nasal bone length in the second trimester: how best to define hypoplasia?

Objective: To determine the best measure of fetal nasal bone hypoplasia for trisomy 21 risk assessment in the second trimester.

Methods: This was a prospective, observational study performed at a single institution between February 2003 and December 2005. Fetuses with nasal bone length recorded sonographically between 16 and 20.9 weeks and known karyotype were included. Definitions of nasal bone hypoplasia assessed included: non-visualized nasal bone, nasal bone < 10th percentile, nasal bone < 2.5th percentile, biparietal diameter/nasal bone ratio >or= 10 and >or= 11 and nasal bone multiples of the median (MoM)
Results: A total of 371 chromosomally normal and 11 trisomy 21 fetuses were included. Nasal bone hypoplasia based on MoM was superior to the other measures of hypoplasia for trisomy 21 risk assessment as reflected by a higher positive likelihood ratio. The risk for fetal trisomy 21 was higher with greater degrees of nasal bone hypoplasia.

Conclusion: Hypoplasia, as defined by MoM, is the superior approach when incorporating nasal bone evaluation into fetal trisomy 21 risk assessment in the second trimester.