OBJECTIVES: To compare two-dimensional with three-dimensional ultrasound evaluation of the fetal nasal bone in the second trimester.METHODS: A prospective, non-interventional study was conducted, in 55 singleton fetuses, between 18 and 24 weeks' gestation. Fetal nasal bone length was measured in the midsagittal plane by two-dimensional imaging and in the midsagittal and coronal plane with three-dimensional ultrasound. All three measurements were compared with one another using one-way repeated samples-measures ANOVA and paired samples t-test.RESULTS: The average fetal nasal bone length (mean ± SD) as determined by the three methods was 7.01 ± 0.94 mm for the two-dimensional midsagittal, 6.96 ± 1.34 mm for the three-dimensional midsagittal, and 6.98 ± 1.32 mm for the three-dimensional coronal plane; comparisons between one another were not statistically significant. Unilateral hypoplasia and bifid shape of the fetal nasal bone were detected in 8.2% and 20.4% of cases, respectively, by three-dimensional ultrasound, whereas all cases evaded detection with two-dimensional ultrasound (p < 0.001 and p = 0.001, respectively).CONCLUSIONS: Fetal nasal bone length measured with two-dimensional ultrasound does not differ significantly from three-dimensional measurements. However, three-dimensional ultrasound is superior in detecting unilateral nasal bone hypoplasia or absence and in assessing fetal nasal bone shape. Hence, fetal nasal bone examination in the second trimester should include three-dimensional ultrasound evaluation.