A Study on Determination of Gestational Age by Ultrasonographic Femur Length Estimation

Abstract

Background: Accurate estimation of gestational age (GA) is fundamental to optimal antenatal care, guiding decisions regarding fetal growth assessment, timing of delivery, and management of high-risk pregnancies. Ultrasonography has become the standard method for fetal dating, and femur length (FL) is one of the most reliable biometric parameters, particularly during the second and third trimesters.

Objectives: To determine gestational age using ultrasonographic femur length estimation and evaluate its correlation and accuracy in comparison with gestational age calculated from the last menstrual period (LMP).

Materials and Methods: A prospective observational study was conducted among 200 pregnant women with singleton pregnancies attending the Department of Radiodiagnosis, Kodagu Institute of Medical Sciences, Madikeri, from January 2024 to June 2025. Femur length was measured using standard obstetric ultrasonographic techniques. Gestational age estimated from femur length was compared with gestational age based on LMP and composite fetal biometric parameters. Data were analyzed using SPSS version 25.0. Pearson correlation and Student's t-test were applied, with p<0.05 considered statistically significant.

Results: The mean maternal age was 26.8 ± 4.3 years, and 54% were multigravida. Mean femur length was 52.8 ± 14.7 mm. Gestational age estimated by femur length closely agreed with LMP-derived gestational age (28.4 ± 6.2 vs. 28.6 ± 6.3 weeks; p=0.184). Femur length demonstrated a very strong positive correlation with gestational age (r=0.984, p<0.001). Accurate gestational age estimation within ±1 week of LMP was observed in 82% of cases, with an overall diagnostic accuracy of 95.5%.

Conclusion: Ultrasonographic femur length is a highly reliable and accurate parameter for determining gestational age during the second and third trimesters. It demonstrates excellent correlation with gestational age derived from LMP and composite fetal biometric measurements and should remain an integral component of routine obstetric ultrasonography.