A number of studies have questioned the use of ultrasound for estimated fetal birth weight.
Given its inaccuracy and resulting interventions, this does NOT seem to be a justifiable use of the technology unless co-existing conditions like diabetes are present (even then, some research questions its use).
The strong acoustic shadow behind the femoral or humeral shaft and the visualisation of both cartilaginous ends indicates that the image plane is on the longest axis and is the optimal measurement plane.
At any gestation, if the ultrasound fetal measurements of each parameter are not in agreement, the reason for this difference should be evaluated.
This is preferable to just averaging all values to arrive at an estimated gestational age.
No formula for estimating the fetal 'birth weight' has achieved an accuracy which enables us to recommend its use.
It should be noted that errors of about 10% are reported and that even at this level of accuracy it is disappointing.