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).
However, it does remain common despite the research against it.
Since all of these measurements are strongly related to gestational age, it is not usually important how they are combined.
No formula for estimating the fetal 'birth weight' has achieved an accuracy which enables us to recommend its use.
The reduced accuracy of prediction of gestational age after 20 weeks must be appreciated.
If we assume a fetus is growing normally, biometric measurements are determined by gestational age, and we can estimate the gestational age and thus the due date.
These measurements are more appropriately used in the assessment of fetal growth, particularly in the second half of the pregnancy, than in the assessment of gestational age.
It is, however, an appropriate measurement in the second trimester to demonstrate normal fetal proportions.
The BPD and OFD are measured on a transverse axial section of the fetal head which includes the falx cerebri anteriorly and posteriorly, the cavum septum pellucidum anteriorly in the midline and the thalami.
The BPD is measured from the outer edge of the nearer parietal bone to the inner edge of the more distant parietal bone. The long bones are measured with the bone across the beam axis.