Efficacy and Regional Validation of the Fetal Femur Length to Abdominal Circumference (FL/AC) Ratio as a Gestational Age-Independent Biometric Parameter in Uncomplicated Second and Third-Trimester Pregnancies

Abstract

Background: Ultrasound evaluation of fetal biometry is fundamental to modern antenatal care, serving as the primary diagnostic tool for monitoring fetal growth, estimating fetal weight, and detecting intrauterine pathomorphology. While absolute biometric parameters—including Biparietal Diameter (BPD), Head Circumference (HC), Femur Length (FL), and Abdominal Circumference (AC)—expand continuously throughout pregnancy, their interpretation relies heavily on accurate gestational age assignment. Objective: To systematically evaluate the sonographic FL/AC ratio in healthy, uncomplicated second and third-trimester gestations within a localized clinical cohort, to mathematically test its gestational age independence, and to validate regional reference norms against standard international benchmarks. Materials & Methods: A cross-sectional observational study was conducted utilizing ultrasound registry data from 40 healthy pregnant women (N = 40) presenting for routine antenatal sonographic evaluation at the Bangladesh Institute of Medical Diagnostic Ultrasound and Research (BIMDUR), Dhaka. All subjects were fully de-identified prior to analysis. Standardized sonographic measurement protocols were executed using a 3.5–5.0 MHz convex transducer. Results: The mean maternal age across the overall cohort was 22.75 ± 4.67 years (range: 15–35 years). The overall mean FL/AC ratio across all 40 subjects was 22.30% ± 1.15% (Range: 20.07% to 23.83%), falling within established normative ranges. Subgroup analysis demonstrated remarkable stability across developmental phases: 2nd Trimester Subgroup (n1 = 24): Mean FL/AC = 22.29% ± 1.15% (Range: 20.07% – 23.65%). 3rd Trimester Subgroup (n2 = 16): Mean FL/AC = 22.31% ± 1.19% (Range: 20.14% – 23.83%). Comparative hypothesis testing confirmed no statistically significant difference in mean FL/AC ratios between the second and third trimesters (t = 0.052, df = 38, p = 0.959). Linear regression analysis plotting the FL/AC ratio against Actual Ultrasound Age in days (103 to 271 days) yielded a near-horizontal regression line (y = −0.0026x + 22.75) with a negligible coefficient of determination (R2 = 0.008, p = 0.584). This proves that less than 1% of the total variance in the ratio is attributable to advancing gestational age. The empirical 5th and 95th percentile bounds for the local cohort were established at 20.25% and 23.68%, respectively. Conclusion: The sonographic FL/AC ratio in this healthy local cohort demonstrates robust constancy (22.30% ± 1.15%) across the second and third trimesters and aligns with established global normative reference thresholds (20.0% – 24.0%). These findings confirm that the FL/AC ratio functions as a reliable, age-independent diagnostic index that eliminates population bias, fully validating the clinical application of default ultrasound software algorithms for routine antenatal screening in local clinical practice.