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. Aim of the Work: was fine out an accurate method for assessment of gestational age in third trimester of pregnancy comparing the Transcerebellar diameter, Biparietal diameter and femur length. Patients and Methods: a total of 200 pregnant women in third trimester pregnancy were assessed in this prospective observational study held at Al-Hussein University Hospital and New Cairo Police Hospital using two-dimensional ultrasound technique during the period from January to June 2018 in order to assess an accurate method for assessment of gestational age in third trimester of pregnancy. Examinations were performed after a written consent received from the patients lying in the dorsal supine position. Two-dimensional ultrasound examination was carried out. Fetal biometry and amniotic fluid volume were assessed. These ultrasound parameters were done under the supervision of a certified gynecologist to exclude bias. Results: the majority of the patients in our study belong to the age group 20 to 30 years (65%) with minimum of 15 years and maximum of 35 years. In our study, also 55 patients out of 200 were primigravida and 145 patients were multigravida. Our patients showed that the percentages of accurate assessment of gestational age within 3 days were as follow: Out of 200 patients, TCD gave accurate assessment within 3 days in 118 patients (59%), FL gave accurate assessment within 3 days in 92 patients (46%), while the BPD gave accurate assessment within 3 days in 59 patients (29.5%). Conclusion: we concluded that TCD is the most accurate method for the assessment of gestational age in third trimester followed by FL, and the least accurate is the BPD. Also by combining accuracy of TCD (90%) and that of FL (80%).