Review of third trimester abdominal circumference (AC) measurements

Ellen Dyer, Trish Chudleigh, Rosie Ultrasound Cambridge University Hospitals


To pilot a method of peer-review for obstetric ultrasound.


Sonographers are aware of the need for peer-review as advocated by both Royal College of Radiologists and BMUS. Vigorous review processes are in place for nuchal translucency scans. Similar peer-review processes are not routine within other areas of obstetric ultrasound. To pilot obstetric peer-review we have chosen third trimester growth scans, and specifically AC measurements due to the implementation of the “Saving Babies Lives” care bundle. This relies on serial third trimester growth scans to monitor pregnancies at risk of fetal growth restriction. AC measurements are the best predictor of fetal wellbeing. There is evidence to suggest that the two-diameter method of measurement is more reproducible than an ellipse.


22 growth scans (5% March 2017) were randomly selected. The BMI, amniotic fluid index and gestational age was recorded. 9 sonographers blindly re-measured 5 AC images using the ellipse and two-diameter methods.  The sonographers were given a PowerPoint presentation of 22 measured AC images to assess independently against image criteria based on the Fetal Anomaly Screening Program.

The results were then presented and discussed with the sonographers.


Preliminary results showed comparable inter-operator variability between ellipse and two-diameter measurements (Table 1).

Minimum Difference ellipse = 16.9mm

Minimum Difference 2-diameter =12.7mm

Maximum Difference ellipse = 33.7mm

Maximum Difference 2-diameter = 35.7mm

The average retrospect ellipse measurements for each case varied from the original measurement obtained during live scanning by between -9.9mm and +1mm, this indicates a possible tendency to over-measure the AC during scans.


Individual sonographers are now more aware of their own practice and the potential for bias.  The next stage will be to evaluate the interpretation of AC measurements by reviewing reports.   We will re-audit in 3 months’ time and plan to use the same model to review other aspects of obstetric ultrasound.

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