This research aimed to determine the efficacy of Mean Gestation Sac Diameter (MGSD) by ultrasound in estimating the Gestational Age (GA) compared to GA by Naegele’s rule using Last menstrual period (LMP) date. This was a prospective observational study of women with a normal spontaneously conceived viable singleton pregnancy, a regular menstrual cycles, and spontaneous onset of labor at term. The LMP was considered certain in all cases. We used ultrasound to scan 2067 fetuses (894 healthy women) and we had 515 MGSD measurements. Data were collected prospectively and used for statistical analysis. We used Descriptive Statistics to calculate the Mean, Standard Deviation (SD), Median and Percentiles values (3rd, 5th, 10th, 50th, 90th, 95th, and 97th) for MGSD measurements on gestational age. We found a regression equation to estimate the GA using MGSD measurements. The results of the current study were compared with different studies using the Paired Differences (t-test analysis). The Mean Sum of Squares of regression deviations of the GA regression model using (MGSD) was 897.9 and this value is significant at P< 0.001. The standard error of the Estimate (Standard Error) and the standard deviation (SD) for the GA regression model (using MGSD measurements) was 0.74 and 0.73 when the GA is< 12 weeks to 0.82 when the GA is between 12-18, respectively.