Embryo measurements dating
The gestational sac is measured in three dimensions, and the average, the Mean Sac Diameter (MSD) used for estimating gestational age.
It is useful between 5 and 8 menstrual weeks with accuracy of /- 3 days .
This is preferable to just averaging all values to arrive at an estimated gestational age.
The wide normal range of BPD in late pregnancy must be appreciated.
Doing an ultrasound to estimate fetal weight near term is a very common practice, one still employed by many OBs, especially with large mothers.
However, research clearly shows that this is a very questionable practice.
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.
If the ultrasound measurements are in agreement and differ from menstrual dates by more than one week prior to 20 weeks a new estimated due date should be calculated and recorded.
It should be noted that errors of about 10% are reported and that even at this level of accuracy it is disappointing.
It is suggested that there may be an improvement in accuracy of about 5% in using three rather than two parameters.
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.