RESUMEN
The ultrasonic measurement of the fetal femur length is a sensitive and precise variable for estimation of fetal growth and development. The objective of this study was to predict gestational age in fetuses older than twenty-four weeks of gestation by ultrasonic measurement of the femur length. In this study, pregnant mothers were identified by the criteria of normality, such as well-known LMP, regular menstrual cycles, no use of oral contraceptive pills for the prior 3 months, no smoking, no history of diabetes. The relation between gestational age and fetal femur length was determined by cross-sectional analysis of 900 normal fetuses [>/= 25 weeks] using realtime ultrusonography. Mathematical modeling of the data demonstrated that the femur growth curve is always linear beyond 24 weeks of gestation. The following regression equation was derived: GA [week] = 5.2 FL [cm] + 2, SD +/- 5 days [Honarvar's Formula 2]. According to this data, the error in estimation of GA for given FL is less than 6 days. This equation appears to be clinically reliable and easy to use. Previous normal ultrasonic fetal femur length curves for other populations may underestimate or overestimate normal fetal age for Iranian population