A Nomogram of Fetal Nasal Bone Length at 16-24 Weeks of Gestation in North Indian Pregnant Women: A Prospective Study.

Introduction Fetal nasal bone length (NBL) is a crucial sonographic marker in prenatal screening for chromosomal abnormalities, particularly trisomy 21. However, its diagnostic accuracy is influenced by ethnic variations, and normative data for North Indian populations remain scarce. Objective This study aimed to establish a gestational age-specific nomogram for fetal NBL between 16+1 and 24 weeks of gestation in North Indian pregnant females, and to evaluate its correlation with standard fetal biometric parameters. Methods A prospective observational study was conducted on 438 structurally and chromosomally normal North Indian pregnant females. Fetal NBL was measured using mid-sagittal ultrasound imaging, and participants were stratified into two gestational age groups, A and B. Group A includes 16-20 weeks of gestational age, and Group B includes more than 20 to 24 weeks of gestational age. Percentile distributions were calculated, and correlations between fetal NBL and other biometric parameters (biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), femur length (FL)) were assessed. Results Mean fetal NBL increased significantly with gestational age (4.56 mm in Group A vs. 5.74 mm in Group B; p < 0.001). Strong positive correlations were found between fetal NBL and BPD (r = 0.926), HC (r = 0.903), and AC (r = 0.874), while a moderate correlation was noted with FL (r = 0.402). The fifth-percentile thresholds for fetal NBL were 3.70 mm (Group A) and 5.10 mm (Group B). Conclusion This study provides a population-specific nomogram for fetal NBL in North Indian pregnancies during the second trimester. The findings underscore the importance of using ethnicity-specific reference values to improve the accuracy of prenatal aneuploidy screening and to reduce false-positive diagnoses.