Background: The provision of obstetric and neonatal care, as well as the public health monitoring of pregnancy outcomes, relies upon the accurate determination of gestational age. Uncertain gestational age has been associated with adverse pregnancy outcomes including low birth weight, preterm delivery and perinatal mortality, independent of maternal characteristics. The use of ultrasonography has significantly improved the evaluation of fetal growth and development and has permitted prenatal diagnosis of a variety of congenital malformations. Objective: The aim of this study was to detect an accurate method for assessment of gestational age in the third trimester of pregnancy comparing the transcerebellar diameter (TCD), biparietal diameter (BPD) and femur length (FL). Patients and Methods: A total of 200 pregnant women in third trimester pregnancy were assessed in this randomized controlled study in Department of Obstetrics and Gynecology at Sohag General Hospital by two-dimensional ultrasound from January to June 2o18 to assess an accurate method for assessment of gestational age in third trimester of pregnancy. Results: We found that out of 200 pregnant women, the TCD gave a correct assessment of gestational age within 3 days in 129 (64.5%) and within 1 week in 178 (89%). While the FL gave a correct assessment of gestational age within 3 days in 97 (48.5%) and within 1 week in 163 (81.5%). The least accurate was the BPD that gave a correct assessment of gestational age within 3 days in 65 (32.5%) and within 1 week in 123 (61.5%) pregnant women. Conclusion: It could be concluded that TCD is the most accurate method for assessment of gestational age in the third trimester followed by FL, and the least accurate is the BPD.