Aim: To compare the accuracy of biparietal diameter and transcerebellar diameter in measuring the gestational age during the third trimester of pregnancy. Material and methods: In this prospective observational study, 275 pregnant women with singleton pregnancy between 32 and 37 weeks gestational age were recruited from the outpatient clinics of both Helwan University Hospital and Ain Shams University Maternity Hospital, Cairo, Egypt, during the period from February 2021 to August 2021. Transcerebellar and biparietal diameters of the fetus were measured by a radiologist blinded to the women’s gestational age, and compared to the gestational age acquired from a reliable date of first day of last menstrual period. Results: The gestational age calculated by first day of last menstrual period ranged from 32 to 37 weeks (34.35 ± 1.4), while estimated by transcerebellar and biparietal diameters ranged from 31 to 37 weeks (34.31 ± 1.39) and 31 to 39 weeks (34.32 ± 1.44), respectively. There was a strong positive correlation between gestational age and transcerebellar diameter (r = 0.98, p <0.001) as well as biparietal diameter (r = 0.87, p <0.001), yet a stronger correlation was with transcerebellar diameter. 93.6% of gestational age assessment by transcerebellar diameter was correct compared to only 79.9% by biparietal diameter. Conclusions: Transcerebellar diameter is a reliable single sonographic fetal biometric parameter for the assessment of gestational age in third trimester of pregnancy.