Background: Accurate gestational age (GA) determination is essential for pregnancy management. Historically, last menstrual period (LMP) has been used, but its reliability is compromised by recall inaccuracies and menstrual cycle variations. Objective: This study aimed to identify the optimal sonographic parameter, transcerebellar diameter (TCD) and biparietal diameter (BPD), or femur length (FL), for third-trimester GA estimation, using LMP as a reference. Patients and methods: A prospective comparative study was conducted on 60 pregnant women in their third trimester. GA was estimated using TCD, BPD, and FL measurements obtained via ultrasound. These estimates were then compared to the GA calculated from LMP to determine the most accurate method. Results: Significant positive correlations were found between TCD, BPD, and FL and GA (P<0.001). Linear regression confirmed each parameter as an independent GA predictor (P<0.001). TCD demonstrated the highest predictive accuracy (AUC=0.902), followed by FL (AUC=0.811) and BPD (AUC=0.796). Specifically, TCD at a cutoff of > 4.79 mm showed 94.5% sensitivity and 80% specificity. FL at > 7.59 mm yielded 61.82% sensitivity and 60% specificity, while BPD at > 9.13 mm showed 81.82% sensitivity and 60% specificity. Conclusion: Transcerebellar diameter (TCD) is the most accurate sonographic parameter for third-trimester GA estimation, surpassing femur length (FL) and biparietal diameter (BPD). Combining TCD, FL, and BPD improves GA accuracy, particularly beneficial in populations with limited medical records or unreliable LMP recall.