Ultrasonic measurement of the femur length as an index of fetal gestational age.

Fetal femur length was measured with a linear-array, real-time ultrasound scanner in 145 uncomplicated gravid patients from 16 to 42 weeks' gestation. The data were analyzed by means of the method of least-squares linear regression. The results of this analysis generated the following equation: Y = -0.6809 + 0.2127 X (Y = femur length; X = gestational age.) With the use of these data, the error in estimation of gestational age given femur length in the interval 25 to 35 weeks' gestation is less than 5 days. Outside these intervals, there is less precision in this measurement, with a range from 8 days at 15 weeks to 6 days at 40 weeks. Furthermore, the correlation coefficient of gestational age versus fetal femur length is statistically greater than that of the gestational age versus fetal biparietal diameter. These results suggest that the measurement of the fetal femur length is a more precise index of gestational age than is the biparietal diameter. We propose that the ultrasonic measurement of the fetal femur length is a sensitive and precise variable for estimation of fetal growth and development.