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.
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.
At any gestation, if the ultrasound fetal measurements of each parameter are not in agreement, the reason for this difference should be evaluated.
It is not expected that BPD be used to assess gestation late in pregnancy.
The values from 33 weeks are intended to predict the growth in fetal head size from a known gestation.
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.
The calipers are placed along the diaphyseal shaft excluding the epiphysis.