Variation of Fetal Nasal Bone Length in Second‐Trimester Fetuses According to Race and Ethnicity

Objective. The purpose of this study was to determine the influence of race and ethnicity on the expected nasal bone length (NBL) based on biparietal diameter (BPD) measured in second-trimester fetuses. Methods. We searched our ultrasound, obstetric, and cytogenetic databases for all second-trimester fetuses with measured NBLs. Fetuses with Down syndrome were identified and excluded from the analysis. Linear regression curves were generated for NBL by BPD according to race and ethnicity. Categories used were African American, Hispanic, Asian, and white. Analysis of variance was used to compare mean variation of observed from expected NBL by BPD according to race and ethnicity. Results. There were 717 fetuses with NBL-by-BPD pairs who were available for analysis in our population, including 139 African American, 58 Hispanic, 22 Asian, and 498 white fetuses. Nasal bone length was highly correlated with BPD for each race (P <.001). Mean variances of observed from expected NBL by BPD were statistically different according to race or ethnicity (P =.0092). Conclusions. Race and ethnicity significantly affect the mean regression line of expected NBL by BPD among fetuses in the second trimester. Genetic sonographic norms, therefore, appear to require race-and ethnicity-specific formulas for NBL.