OC18.01: the Prenasal Thickness to Nasal Bone Length Ratio: 2D Versus 3D Ultrasound

Recently we showed that the prenasal thickness to nasal bone length ratio (PT:NBL) measured in 3D volumes of the fetal face is a very strong marker for trisomy 21 with a detection rate and positive likelihood ratio of 100 (95% CI, 89–100)% and of 21.2, respectively. Objective of this study were to compare PT:NBL ratio's measured on three-dimensional (3D) volumes and two-dimensional (2D) pictures. PT and NBL measurements were obtained from stored 3D volumes or good 2D pictures of 90 second and third trimester trisomy 21 fetuses. All ultrasound examinations were performed by experienced sonographers and images were obtained by a General Electric Voluson 730 Expert or E8 ultrasound system. In all cases the insonation angle for NBL and PT measurements with respect to the longitudinal axis of the nasal bone was between 45 and 135 degree. The PT:NBL ratio was calculated by dividing PT by NBL. The 3D based 95th percentile (0.80) was used as cut-off value. PT and NBL were measured in 26 3D volumes and 64 2D pictures. In the 3D group the mean gestational age was 23 3/7 (range, 15 1/7-37 6/7) weeks and in the 2D group 21 6/7(range, 14 1/7-36) weeks. The mean PT:NBL ratio in the 3D group or the 2D group was 1.45 (range, 0.80-5.22) and 1.33 (range, 0.62-2.64), respectively(not significantly different, unpaired Student's t-test P=). The detection rate in the 3D group and the 2D group was 100% and 92% (24 out of 25, respectively (not significantly different, Chi-square P=). The PT:NBL is a strong second and third trimester marker for trisomy 21 measured. The marker can equally reliably measured with either 3D or 2D ultrasound.