OP09.09: Use of 3D ultrasound in visualization and measurement of nasal bone length and prenasal thickness in normal second and third trimester fetuses

Abstract

Objectives: To create reference ranges for nasal bone length (NBL) and prenasal thickness (PT) based on measurements performed on 3D to the exact median plane corrected profile views and to examine their mutual relationship in normal second and third trimester fetuses. Methods: 3D profile volumes of 200 normal fetuses at 15–33 weeks gestation were reviewed and adjusted by multiplanar mode to the correct median profile. The nasal bone was measured from the nasion to the most anterior end of the ossification line. The frontal bone extends behind the nasal bone, therefore care was taken not to add this part of the frontal bone to the measurement of NBL as this would erroneously increase the measurement. The PT was measured as the shortest distance between the nasion and the frontal skin. Results: Good visualization of the landmarks and good NBL and PT measurements were obtained in 109 cases and 108 cases, respectively. In 106 cases, both NBL and PT could be measured in the same profile. NBL increased with gestational age, from 3.2 mm at 15 weeks to 9.0 mm at 33 weeks (NBL followed a second order polynomial trend). PT increased with gestational age from 2.3 mm at 15 weeks to 6.1 mm at 33 weeks (PT followed a first order polynomial trend). NBL and PT were highly correlated (r = 0.83, P < 0.001). Linear regression analysis showed that NBL contributes to 46% of PT increase (r = 0.46, P < 0.001). Conclusions: These are the first reference range for NBL and PT constructed on by multiplanar view corrected exact median profiles of normal second and third trimester fetuses. The range extends to the third trimester and both parameters are measured in the same fetus. Redefinition of the measurement technique for the NBL by excluding erroneous inclusion of part of the frontal bone, produces systematically smaller measurements than previously published reference ranges. NBL and PT increase linearly with gestation, with less steep increase for the NBL after 28 weeks.