Background: Accurate pregnancy determination is one of the most useful assessments of pregnancy, which depends on whether or not the patient is pregnant. Objective: To estimate gestational age using trans cerebellar diameter in comparison to femur length and biparietal diameter in 2nd and 3rd trimesters of pregnancy according to first day of last menstrual period. Patients and methods: This prospective study included 90 pregnant women. All cases were selected from Obstetrics and Gynecology Department, SayedGalalHospital seeking antenatal care, routine ultrasound during the period from November 2020 till June 2021. The study had been conducted at Obstetrics and Gynecology Department, Faculty of Medicine, Al-AzharUniversity. Regarding the study group, they had been divided into three groups: Group 1: 14-20 weeks (early second trimester), Group 2: 21-29weeks (late second trimester), and Group 3: 30-40 weeks (third trimester). Result: Gestational age by trans-cerebellar diameter (TCD) in women with gestational age was accurate in 92% of patients, while in women with gestational age ranged between 21 – 29 weeks group was accurate in 93.9% of patients, and in women with gestational age ranged between 30 – 40 weeks group was accurate in 90.6% of patients with no statistically significant differences between groups. Conclusion: Trans-cerebellar diameter was more reliable method of gestational age determination in third trimester of pregnancy than biparietal diameter. Trans-cerebellar diameter (TCD) and femur length (FL) can be used as a tool to assist in the assessment of gestational age in third trimester.