Introduction: Femur length (FL) is a fetal biometric parameter used for estimation of gestational age. Gestational age determination by ultrasonography (USG) is very essential as it gives an accurate estimation.

Aim and Objectives: This study is an attempt to measure FL of the fetus for the prediction of gestational age in the IInd and IIIrd trimester of pregnancy by USG. Materials and Methods: Five hundred pregnant women in the IInd and IIIrd trimester with the single live fetus and having no other complications either in mother or fetus, were selected for the study. Fetal FL was measured, and results were analyzed for accuracy in estimation of gestational age and were compared with findings of other workers.

Observation and Results: Gestational age estimated from FL in 500 cases between 20 and 36 weeks of gestation showed that in the mean FL in each week of gestation the maximum difference of ±0.7 mm was found in the IInd trimester and ±2 mm was in the IIIrd trimester. The accuracy of the correctly diagnosed cases and P value decreases as pregnancy advance.

Conclusion: Sonographic examination was found to be effective to assess the accurate FL parameter for different weeks of gestation. FL was found to be more appropriate in predicting gestational age in the IInd trimester and its reliability decreases in the IIIrd trimester.