Nasal-bone length in euploid fetuses at 16-24 weeks'gestation by three-dimensional ultrasound

Objective To establish a normal range of nasal-bone length at 16-24 weeks' gestation by the use of threedimensional (3D) ultrasound and to investigate the effect of deviations from the exact mid-sagittal plane on the measurement of nasal-bone length. Methods We acquired 3D volumes of the fetal profile from 135 normal fetuses at 16-24 weeks' gestation. The multiplanar mode was used to obtain the exact midsagittal plane and to produce parasagittal and oblique views of the fetal face. Nasal-bone length was measured in each plane and the sonographic landmarks of each profile view were examined. Results Nasal-bone length increased with gestational age from a mean of 4.1 mm at 16 weeks to 7.1 mm at 24 weeks. There was a tendency to underestimate nasal-bone length when the measurements were taken in parasagittal planes and to overestimate the measurements when they were taken in oblique views, compared to the exact mid-sagittal plane. The mean difference in nasalbone length from the one in the mid-sagittal plane was 0.42 and 0.63 mm for parasagittal measurements at 1 and 2 mm, respectively, from the midline, -0.08 and -0.51 mm for oblique measurements at 10° and 20° rotation along the z-axis and -0.69 mm for rotation of 20° along the z-axis and 10° along the y-axis. The vomeral bone was the only sonographic landmark defining the exact mid-sagittal plane of the face that was not visible in the parasagittal and oblique planes. Conclusions Parasagittal and oblique scanning planes may produce different degrees of under-or overestimation of nasal-bone length compared to measurements systematically taken in the exact mid-sagittal plane. Inclusion of the vomeral bone in the definition of the exact mid-sagittal plane of the face could improve the reproducibility of measurements of nasal-bone length.