Predictive role of prenasal thickness and nasal bone for Down syndrome in the second trimester

To assess the efficacy of prenasal thickness (PNT) and nasal bone (NB) for prediction of Down syndrome (DS) fetuses in the second trimester ultrasound examination.

PNT was measured from stored two-dimensional fetal profile images taken at 15-23 weeks in 242 fetuses with normal karyotype (Group 1) and 24 fetuses with DS (Group 2). It was measured as the shortest distance from the anterior edge of the lowest part of the frontal bone to the skin. The efficacy of PNT, NB, PNT/NB and biparietal diameter (BPD)/NB was evaluated for prediction of DS.

PNT values increased with gestational age in normal fetuses. PNT measurement was ≥95th percentile in 54.2% (13/24) of the DS cases and 2.9% of the normal cases. Receiver operator curve analysis showed that PNT/NB ratio had the best area under the curve with a detection rate of 80% for a false positive rate of 5% at a cut-off value of 0.76.

PNT is increased in fetuses with DS as compared to normal fetuses. PNT/NB≥0.76 in the second trimester is a better predictor of DS than the use of PNT or NB alone.