OC06.08: Use of new second trimester craniofacial markers of Trisomy 21 at 20 to 25 weeks screening ultrasound

To evaluate the performance of new craniofacial markers for prediction of Trisomy 21 (T21) at second trimester screening ultrasound. This case-control study included singleton pregnancies with a 20 to 25 weeks screening ultrasound at the Fetal Medicine Unit of Hospital Clínico de la Universidad de Chile between 2004 and 2017. The following markers were included: gestational age, biparietal diameter (BPD), prenasal thickness (PNT) and nasal bone length (NBL). PNT/NBL and BPD/NBL ratios were calculated. For comparison between euploids and T21 fetuses, the Mann-Whitney U test was used and expressed as median (interquartile range). Finally, local reference curves were generated for PNT and BPD/NBL, according to Royston methods. Sensitivity, false positive rate and positive likelihood ratios were obtained in PNT and both ratios. Out of a total of 11,356 fetuses assessed, 37 (0.3%) were diagnosed with T21. Among them, 3,875 had prenasal thickness (PNT) measurements and 9,355 had BPD and NBL measurements. PNT, PNT/NB, and BPD/NBL ratios were higher in fetuses with T21 compared to euploid fetuses (PNT = 5.2 [4.3-5.7] mm vs. 4.0 [3.5-4.4] mm, p < 0.001; PNT/NBL = 1.03 [0.81-1.70] vs. 0.65 [0.57-0.75], p < 0.001; BPD/NBL = 12.5 [9.6-21.5] vs. 9.3 [8.4-10.4], p < 0.001). PNT showed a direct correlation with gestational age, following a linear curve. The 95th percentile at 20 and 25 weeks was 4.5mm and 5.6mm, respectively. Sensitivity, positive predictive value, and relative risk of PNT > 95th percentile were 55%, 4%, and 10, respectively. For PNT/NB > 0.93, they were 65%, 5%, and 13, respectively. Finally, for BPD/NBL > 95th percentile, they were 59%, 8%, and 8, respectively. The new craniofacial markers for T21 in second trimester ultrasound showed promising performance as standalone parameters. However, their utilization necessitates the development of reference curves for each marker based on the local population.