Defining nasal bone hypoplasia in second-trimester Down syndrome screening: does the use of multiples of the median improve screening efficacy?

OBJECTIVE: The purpose of this study was to compare the efficacy of the use of nasal bone ( NB) multiples of the median ( MoM) with the biparietal diameter ( BPD)/ NB ratio as definitions of NB hypoplasia that is associated with Down syndrome in the second trimester. STUDY DESIGN: We conducted a prospective cohort study of women who underwent an anatomic survey between 16 and 22 weeks of gestation. The fetal NB and other markers of fetal aneuploidy were evaluated. MoMs for the NB length at each gestational age category were calculated and adjusted for maternal race. NB hypoplasia was defined either as an absent NB or by a ratio of the BPD/ NB > 11 or by NB lengths < 0.75, 0.5, and 0.25 MoM for the gestational age, respectively. Fetuses or infants with Down syndrome were compared with those without for the presence of NB hypoplasia. RESULTS: Among 3634 women whose condition was evaluated, NB assessment was obtained in 3197 women ( 88%). There were 23 cases of Down syndrome that were detected. Receiver operating characteristic curve comparison revealed NB MoM < 0.75 to be the best definition of NB hypoplasia ( area under receiver operating characteristic curve, 0.75). NB < 0.75 MoM had a sensitivity and specificity of 49% ( 95% Cl, 26- 69) and 92% ( 95% Cl, 91%- 93%), respectively, compared with 61% ( 95% Cl, 38%- 80%) and 84% ( 95% Cl, 82%- 85%), respectively for BPD/ NB > 11. The difference in the sensitivity of 12% ( 95% Cl, - 5-31) with the BPD/ NB ratio > 11 vs with an NB MoM < 0.75 was not significant ( P <.25). The difference in the specificity of 8% ( 95% Cl, 7.5- 9.5) with the BPD/ NB ratio > 11 vs an NB MoM < 0.75 was significant ( P <.0001). CONCLUSION: In the second trimester of pregnancy, the use of nasal bone length < 0.75 MoM for the gestational age was the best definition for Down syndrome detection and resulted in improved specificity.