Comparison of the ratio of second trimester fetal biometric measurements to fetal nasal bone length in fetuses with normal karyotype and trisomy 21

Aim: In this study, we compared the ratio of second trimester fetal biometric measurements to nasal bone length (NBL) in fetuses with normal karyotype and trisomy 21 to determine their diagnostic prognostic value.

Materials and methods: The study included 148 pregnant women who obtained second-trimester ultrasonographic fetal anatomy and had amniocentesis (AS) for fetal karyotyping. The fetal karyotype results divided the groups into normal and trisomy 21 fetuses. Age, obstetric history, first and/or second trimester screening test risk ratios, fetal biometric measurements, and NBL mm, median (MoM) multiples, and percentile values were recorded and compared between groups.

Results: BPD/NBL ratios above 9.26 predict trisomy 21 in fetuses with 77.6% sensitivity and 86.1% specificity (p = 0.001). HC/NBL ratios above 34.50 predict trisomy 21 in fetuses with 77.8% sensitivity and 88.8% specificity (p = 0.001). FL/NBL ratios above 6.02 predict trisomy 21 in fetuses with 69.6% sensitivity and 72.2% specificity (p = 0.001). HL/NB ratios above 6.56 predict trisomy 21 in fetuses with 95.5% sensitivity and 47.2% specificity (p = 0.001). The NBL MoM value demonstrated a high diagnostic accuracy for normal-karyotype fetuses (p = 0.021).

Conclusion: We found that BPD/NBL, HC/NBL, FL/NBL, and HL/NBL ratios differed between fetuses with a normal karyotype and those with trisomy 21, specifically the HC/NBL ratio, which predicted trisomy 21 with good diagnostic accuracy. In identifying normal-karyotype fetuses, the NBL MoM was highly accurate.