Assessment of gestational age based on ultrasonic femur length after the first trimester: a simple mathematical correlation between gestational age (GA) and femur length (FL)

Abstract

Objective: 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 more than 13 weeks of gestation by ultrasound measurement of the femur length. Method: In this study, pregnant mothers were identified by the criteria of normalities such as: well-known LMP, regular menstrual cycles, no use of OCP for the last 3 months, no smoking, no history of diabetes, etc. Result: The relation between gestational age and fetal femur length has been determined by cross-sectional analysis of 900 normal fetuses (≥14 weeks of gestation) using real time ultrasonography. Mathematical modeling of the data has demonstrated that the femur growth curve is non-linear beyond 13 weeks of gestation. With the aid of a scientific calculator the data were analyzed and a simple second grade equation has been derived: GA (weeks)=0.262 2 FL (cm)+2 FL+11.5, S.D.∼±5 days (Honarvar’s Formula 3). With the use of this data, the error in the estimation of GA given as FL is ±5days. Conclusion: This simple, new and accurate equation appears to be clinically reliable and easy to use and suggests that previous normal ultrasonic fetal femur length curves for another population may under- or overestimate normal fetal age for the Iranian population. Thus, our formula is an excellent means of estimating true gestational age.