Background: Normal development of placenta during gestation is necessary for supporting of a healthy fetus. On the other hand, any impairment in its development may have a profound impact on fetal development and pregnancy outcome. Objective: To study the placental thickness by ultrasonography in relation to gestational age (GA). Patient and Methods: The present prospective study was conducted on 100 pregnant females attending antenatal clinics in both Damanhour Educational Hospital and Eldelngat central Hospital in the period from August 2018 to August 2019. The placental thickness in mm was measured at the level of cord insertion site. Placental thickness was calculated from the echogenic chorionic plate to placental myometrial interface near the mid-placental portion. Results: There was a positive correlation between increase in placental thickness (mm) in total sample, 2nd trimester, and 3rd trimester and GA, as GA increase Placental thickness increase. Conclusion: Placental thickness measured at the level of umbilical cord insertion can be used as an accurate sonographic indicator in the assessment of gestational age because of its linear correlation. Therefore, it can be used as an additional sonographic tool in correlating gestational age in cases where last mestrual period (LMP) is not known and in detecting patients developing intrauterine growth retardation (IUGR).