Background: The provision of obstetric and neonatal care and pregnancy outcomes relies upon the precise determination of gestational age. Many patients in our setup due to socio-economic reasons come for their first antenatal visits in third trimester. There is a crucial requisite for programmatically feasible and accurate approaches of gestational age dating in low and middle income countries. Trans cerebellar diameter (TCD) has evolved as a promising indicator for assessing fetal growth and gestational age. Objective: This study aimed to compare between the accuracy of fetal transcerebellar diameter and biparietal diameter in gestational age measurement in the third trimester of pregnancy. Patient and methods: This study comprised 200 pregnant women at third trimester who were chosen from the outpatient clinics at El-Sayed Galal Hospital of Al-Azhar University during the period of research from May 2020 to December 2020. They all had singleton uncomplicated pregnancy at third trimester pregnancy between 28th week of gestation and term (gestational age (GA) evaluation was based on menstrual history (LMP) and early ultrasonographic assessment before 15 weeks of gestation). Additionally, they had sure date of last normal menstrual period (LNMP) and they all had viable fetus in the longitudinal lie, cephalic presentation. Results: The overall accuracy of estimated gestational age based on various fetal biometric parameters within three days from that calculated via last menstrual period (LMP) was: the highest accuracy was reported in GA-TCD in 55.5 % of cases followed by GA- femur length (FL) & GA- head circumference (HC) that were accurate in 42.5% of cases then GA- abdominal circumference (AC) was accurate in 32 %, and the least was GA- Biparietal diameter (BPD) was accurate in 31.5%. While when detecting accuracy within one week, the highest accuracy was reported in GA-TCD in 83 % of cases followed by GA-FL that was accurate in 76% of cases then GA-HC was accurate in 71.5% GA-BPD was accurate in 65.5%, and the least was GA-AC that was accurate in 57 %, and of cases. Conclusion: It is concluded that TCD had shown better correlation and predictive accuracy of gestational age determination in third trimester of pregnancy then femur length they were superior to biparietal diameter and with the femur length, TCD can be utilized as a tool to aid in the assessment of gestational age in third trimester. As this is essential particularly notably in our country as many of our patients attend the hospitals with lacking medical record or prior antenatal care visits especially in low socioeconomic rural areas, not recalling their LMP.