P190: Audit of the sonographic estimation of fetal weight based on fetal abdominal circumference

Abstract

To determine if estimated fetal weight was within ± 10% of the actual birthweight and to determine if the margin of error was dependent on actual birthweight or gestation. This audit was performed prospectively. Sixty-four fetuses were identified who had an ultrasound scan within one week of delivery. The notes were reviewed for the gestation, abdominal circumference, measurement to birth interval and the actual birthweight. The abdominal circumference was used to calculate the estimated fetal weight using the Campbell and Wilkin equation as per department protocol. The percentage difference between estimated and actual birth weight, the mean and standard deviation (SD) was calculated. There was an overestimation of the actual birthweight in 83% of cases. In this study 45% of estimated fetal weight deviated more than 10% from the actual weight. Fetuses of birthweight less than 2.500 gms comprised 45% of these cases. In this sub-group of fetuses (< 2500 gms) the estimated fetal weight deviated by more than 10% in 62% of cases and the mean (± SD) percent deviations from the actual birthweight was 19 ± 10.5%. There was no difference in percentage error according to gestation, measurement to delivery interval or sonographic abdominal circumference. There is marked overestimation of the actual birthweight based only on the ultrasound abdominal circumference with the greatest percentage error at fetal weight less than 2500 gms. Usage of the ellipse technique of measurement and combination of abdominal circumference with femur length in the Hadlock equation may decrease the deviation.