Objectives:To assess the efficacy of prenasal thickness measurements on second trimester ultrasound examination in prenatal prediction of Down syndrome fetuses. Methods: Prenasal thickness was measured from stored fetal profile images during 16-24 week second trimester scans. Images from 115 women with normal fetuses and 15 women with Down syndrome were included. Prenasal thickness was measured as the shortest distance from the anterior edge of the lowest part of the frontal bone to the skin. Delta values for each gestational week for prenasal thickness were calculated for statistical analysis. Results: In the normal group prenasal thickness increased with gestation (prenasal thickness = −28.747 + (2.254 × GA, R2, P < 0.01). There was a statistically significant increase in the mean prenasal thickness measurement in Down syndrome fetuses. The prenasal thickness measurement was above the 95 th centile in 66.7% (10/15) of all Down syndrome cases, including 4 of 5 with an absent nasal bone and 6 of 10 with a nasal bone length above 2.5 mm. Conclusions: Prenasal thickness is increased in fetuses with Down syndrome as compared to normal fetuses. Prenasal thickness may be an additional predictor for Down syndrome on the second trimester ultrasound. Objectives: To determine if sonographic dimensions of the facial profile on a population of fetuses with absent nasal bone, differs from the normal population. Methods: 16 fetuses with absent nasal bone and with an adequate volume dataset of the fetal profile where included in the study. After a multi-planar manipulation to obtain a mid sagittal plane of the profile, 3 different operator measured the distance between the tip of the nose and the mouth (A), between the mouth and the gnathion (B), between the upper philtrum and the mouth (a) and between the mouth and the upper concavity of the chin (b), as recently reported by Goldstein et al, 2010. The values were plotted between the reported confidence limits, analyzing its percentage of distribution when it was corresponding or not to trisomy 21 (T21). Prenasal thickness was also measured. Wilcoxon non parametric test was used to determine differences between groups of measurement. Results: 2 cases were excluded (gestational age more than 26+6 weeks). The mean gestational age of the remaining was 21+3 (range, 18+2-26+5). 10/14 (71.5%) were carriers of T21 and the remaining were fetuses with normal karyotype. The A, B and (a) measurements were registered between the reported intervals of confidence, without statistical differences between both groups of fetuses with/without nasal bone. 60% of the fetuses with T21 had (b) measurement below the interval of confidence. The same measurement, on normal karyotype/absent nasal bone fetuses, were in the normal interval. Concordance between the three operators was found for the measurements, particularly (b). Mean prenatal thickness was 6.26 (T21) and 3.9 mm (non T21). OP09.06 Conclusions:Although the sample is small and ideally must be compared with our own population of healthy fet...