Objective: to evaluate how well biparietal diameter and transcerebellar diameter performed in estimating the gestational age of pregnant women in their third trimester. Study Design: Cross-sectional study. Place and Duration of Study: Armed Forces Institute of radiology and Imaging, Rawalpindi Pakistan, Feb to Aug 2022. Methodology: There were 120 pregnant women who went to the Obstetrics and Gynecology Departmental walk-in clinic or Emergency room between weeks 28 and 40 of their pregnancies. Ultrasound was done on all patients that had been preselected after a complete history and physical examination had been completed; transcerebellar diameter (TCD) and biparietal diameter (BPD) were measured and compared with LMP. Results: Mean age of the participants recruited in the study was 27.08±1.45 years. There was a significant increase in mean BPD from 28 weeks of gestation (68.01±7.4 mm) to 36 weeks of gestation (89.77±2.35 mm). During 28 weeks of gestation, the mean TCD was 30.3±1.49 mm, whereas at 36 weeks it peaked at 48.1±1.21 mm. By looking at the median gap between real and estimated GA by BPD, we find that when real GA rises, the magnitude of the age estimation error decreases significantly. The error was 3.22±0.17 days for GA at 28 weeks, 2.48±0.09 days at 34 weeks, and 2.18±0.01 days at 36 weeks. A statistically significant (p<0.001) shift in the mean error was observed. ... Conclusion: Both TCD and BPD were shown to be helpful in this study's context, however statistically speaking, TCD was superior to BPD. ...