Nasal bone in first-trimester screening for trisomy 21

Objective This study was undertaken to investigate the impact of incorporating assessment of the nasal bone into first-trimester combined screening by fetal nuchal translucency (NT) thickness and maternal serum biochemistry. Study design In this prospective combined screening study for trisomy 21, the fetal nasal bone was also examined and classified as present or absent. A multivariate approach was used to calculate patient-specific risks for trisomy 21 and the detection rate (DR) and false-positive rate (FPR) were estimated. We examined 2 screening strategies; first, integrated first-trimester screening in all patients and second, first-stage screening of all patients using fetal NT and maternal serum free ß-hCG and PAPP-A, followed by second-stage assessment of nasal bone only in those with an intermediate risk of 1 in 101 to 1 in 1000 after the first-stage. Results The nasal bone was absent in 113 (0.6%) of the 20,165 chromosomally or phenotypically normal fetuses and in 87 (62.1%) of the 140 fetuses with trisomy 21. With combined first-trimester NT and serum screening, the DR of 90% was achieved at a FPR of 5%. Inclusion of the nasal bone, either in all cases or in about 10% of the total in the 2-stage approach, halved the FPR to 2.5%. Conclusion Inclusion of the nasal bone in first-trimester combined screening for trisomy 21 achieves a DR of 90% for a FPR of 2.5%.