This research aimed to determine the efficacy of Head Circumference (HC) by ultrasound in estimating the Gestational Age (GA) compared to GA by Naegele’s rule using Last menstrual period (LMP) date. Materials and Methods: 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 1553 Head circumference (HC) 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 HC measurements on gestational age. We found a regression equation to estimate the GA using HC measurements. The results of the current study were compared with different studies using the Paired Differences (t-test analysis). These results can be useful in women who cannot recall their last menstrual period (LMP). Our criteria will provide useful references for estimating gestational age and fetal care. A larger study might be needed to include a larger sample of the population.