Background: It is important to detect gestational age in order to better pregnancy and fetal outcome management. Several biometric measures were indicated to be reliable in measuring gestational age. Aim of the Work: to assess the accuracy of trans-cerebellar diameter (TCD) measurement in estimation of the gestational age during the second trimester of pregnancy compared to current fetal biometric measurements of femur length (FL) and bi-parietal diameter (BPD) according to last menstrual period (LMP). Patients and Methods: This observational study included 200 cases in their second trimester of pregnancy. They were recruited and assessed for eligibility from at the outpatient Clinic and Causalities of the Obstetrics and Gynecology Department, El-Monira general hospital, Cairo. Results: a significant differences were found when comparing GA-LMP with each of GA-TCD (p value=0.038), GA-FL and GA-BPD. Our results indicated that regarding the gestational age detected according to the TCD and comparing it with the gestational age determined by FL and BPD, they were statistically highly significantly different. Regarding the accuracy rate of the biometric parameters within one week, our results indicated that TCD showed the highest accuracy rate (98.5%) in compare with BPD (95.5%), and FL (94.0%) accuracy. Results also indicated the presence of significant correlations between all biometric measurement and their gestational age estimation. On the other hand we found that the TCD had the highest correlation in comparison to the other biometric measurements. Conclusion: We conclude that trans-cerebellar diameter (TCD) is more accurate measurement in detecting gestational age during second trimester of pregnancy.