The accurate gestational age is critically important for pregnancy management from the first trimester to delivery, and is particularly necessary for determining viability in premature labour and in post-dates deliveries. Prior to the widespread use of ultrasound, caregivers relied on a combination of history and physical examination to clinically determine gestational age. Ultrasound gave clinicians a method to measure the fetus and therefore to estimate gestational age. This was across section study to evaluate the gestational age measuring the Occipito Frontal Diameter in the second and third trimester of pregnancy, and correlate with the gestational age estimated by Last Menstrual Period, Biparietal Diameter and Femur Length. 483 Sudanese Pregnant women underwent routine sonographic examination at different antenatal care units at Khartoum state using 3.5MHz curve liner transducer; The Occipito Frontal Diameter is measured perpendicular to the Biparietal Diameter. The statistically significant strong relationship between OFD and GA. Gestational age can be depicted by the following equations: GA=3.663+ (.286*OFD) with SR=2.882. Measuring of OFD was useful for the assessment of gestational age. Accurate prediction of gestational age was obtained by combination of different parameters.