Sonographic measurement of the fetal femur: factors affecting accuracy

Variations have been observed in the measurement of fetal femoral diaphysis length by ultrasound. This in vitro study examines the effect of depth, the angle of the bone relative to the axis of the sound beam, the machine, and the observer on the precision of measurement. Using analysis of variance techniques, it demonstrates that three of these factors (depth, angle, and machine) have significant effects. Depth-associated variations were less than one millimeter and are probably not of clinical importance. However, the variations due to changes in angle are highly significant (P = less than 0.001) and may be controlled by obtaining the measurement with the femur perpendicular to the sound beam. Errors due to variation between machines were also highly significant (P = less than 0.001). When determining serial growth patterns, the errors due to variations between machines can be controlled by using the same machine for each examination. The variation between machines is also an important factor to consider when choosing a curve for reference purposes.