Introduction: Cervical disability for preservation of pregnancy is called Cervical insufficiency. On time diagnosis of Cervical insufficiency is important for preventing actions. Endovaginal sonography is one of the most important methods. The importance of Cervical Length measurement is to diagnose Cervical insufficiency and to decide on cerclage. In this study, we evaluated endovaginal sonography assessment of the Cervical Length changes during a normal pregnancy.Materials and Methods: In a prospective cross-sectional study, endovaginal sonography was performed to measure the Cervical Length and the mean of the Cervical Lengths on 150 pregnant women (50 in every trimester). They were compared together by the one way Anova test. The patients were followed until labor. Twin pregnancies and cases with a history of uterus curettage, Cervical anomaly, preterm labor and cerclage were eliminated from the study.Results: The mean of the Cervical Length in the first, second and third trimesters of pregnancy were 39 mm, 40.7mm and 39.3mm, respectively. Also, the minimum and the maximum Length of the cervix in the same trimesters were: 28 and 56 mm, 26 and 52 mm, 27 and 52mm, respectively. It was noted to history of normal vaginal delivery (NVD), past cesarean and the first pregnancy. The minimum Cervical Length was seen in the first pregnancy and the maximum Cervical Length was seen in past NVD. Between the means of Cervical Length in three trimesters of pregnancy there was no significant statistical difference, but there was a significant statistical difference between the mean Cervical Length and the maternal parity. On the other hand between maternal age and Cervical Length there was no significant statistical difference.Conclusion: By comparing the mean Cervical Length in three trimesters, 39 mm was accepted for mean Cervical Length in different trimesters. There was a significant statistical correlation between the mean Cervical Lengths and multiparity with no preterm labor history, so it is safer if there is no preterm labor history in a multiparous pregnant woman.