Assessment of gestational age by ultrasonic measurement of the femur length.

The effectiveness of ultrasonic femur length measurement from 12 to 40 week's gestation, as a means of assessing fetal age, was tested and compared with that of biparietal cephalometry. The femur length and biparietal diameter (BPD) were obtained from 471 measurements, from pregnant women with confirmed datings, using real-time scanning with a 3,5 mHz transducer frequency. Using a freeze frame and electronic calipers, the mean value of three consecutive measurements of the femur, when visualized with its characteristic appearance, was recorded. Linear regression analysis with the correlation coefficient of the femur growth-curve (r = 0.989, p less than 0.001) and that of the BPD (r = 0.985, p less than 0.001) showed that the former is as good as the latter. The femur growth-curve from 12 to 40 week's gestation with a mean +/- 2 SD was constructed. Estimation of fetal age by femur length measurement was compared with that assessed by the BPD in a further 54 women. A close correlation was found (r = 0.993, p less than 0.001). Measurement of the fetal femur appears to be a reliable method for assessing gestational age, which can compensate for the limitations of the BPD method.