Background: In obstetric treatment, it is critical to accurately evaluate the gestational age of the pregnant woman. Low birth weight, spontaneous preterm delivery, and perinatal death have all been linked to pregnancies in which the gestational age was not known. The study's goal is to assess the accuracy of gestational age measurement in the third trimester between trans-cerebellar diameter, bi-parietal diameter, and femur length. The patient and the procedure are both in good hands. From December 2020 to June 2021, a prospective cohort study at the Department of Obstetrics and Gynecology at Imbaba General Hospital and Tabark private hospital evaluated 220 pregnant women in their third trimester using two-dimensional ultrasound to determine an accurate method for calculating gestational age during the third trimester of pregnancy. TCD correctly predicted gestational age in 96.4 percent of 220 pregnant women within 3 days, 97.5 percent within 5 days, and 97.5 percent within 7 days (98.8 percent ). However, in 169 (76.8%), 202 (91.8%), and 211 (91.8%) cases, the FL correctly predicted gestational age within 3 days, 5 days, and 7 days, respectively (95.9 percent ). BPD was the least reliable, correctly determining gestational age within 3 days in just 68% of pregnant women, 95% within 5 days in 199, and 96% within 7 days in 212 pregnant women. For third-trimester gestational age estimation, we may say that TCD is most accurate, followed by FL, and the least accurate, BPD.