Introduction: The accurate dating of pregnancy is critically important for pregnancy management from the first trimester to delivery, and is particularly necessary for determining viability in premature labour and in post-dated deliveries. Prior to the widespread use of ultrasound, caregivers relied on a combination of history and physical examination to clinically determine gestational age.

Objective: To compare the Gestational Age in Third Trimester determined by Using Fetal Femur Length Versus Biparietal Diameter.

Materials and Method: In the present study total 100 pregnant female attending antenatal clinic of the study institute in third trimester were enrolled. Detailed demographic profile including age, address etc was obtained. Detailed history regarding the exact knowledge of her LMP, cycle regularity and duration of cycle was taken. All patients then underwent detailed examination i.e. general and systematic examination. The gestational age at time of scanning was confirmed by LMP, and abdominal examination. All the women in the study underwent ultrasonoghraphy. The ultrasound was done by a single operator to avoid bias in observations. All the fetal parameters were assessed and special attention was given to the femur length and biparietal diameter. The collected data was entered in Microsoft excel. Primary analysis was conducted by using Epi Info statistical software.

Results: The mean gestational age calculated from LMP was 33.53±3.37 weeks. The mean gestational age was calculated by using BPD and femur length on ultrasonoghraphy was 35.87±2.96 weeks and 34. 15±3. 40 weeks respectively. When the gestational age calculated from LMP and BPD was compared it was observed there was statistically significant difference in the two methods. While the gestational age calculated by LMP and FL showed no statistically significant difference. Thus the gestational age calculated from femur length was more accurate as compared to BPD. Similarity of GA in LMP with BDP was observed in 13 patients while similarity with FL observed in 74 patients. The Correlation coefficient between GA by LMP and GA by BDP was 0.9268 and correlation coefficient between GA by LMP and GA by FL is 0.9603. The coefficient between GA by BPD and GA by FL was 0.9099.

Conclusion: The gestational age estimated from Biparietal diameter and fetal femur length were statistically significantly correlated with the gestational age calculated from the LMP. But the gestational age calculated from femur length was more accurate as compared to BPD.

Keywords: Ultrasonography; Gestational Age; BPD; Femur Length.