P29.04: Ultrasound (in)accuracy: it's in the formulae not in the technique: revisiting the accuracy of abdominal circumference measurement in term pregnancies

Abstract

error of estimated fetal weight was 238 g (absolute range 3 g-855 g, real range −855 g-792 g), with a mean absolute percentage error of 6.7% (absolute range 0.08%-21%, real range −21% −20%). Twenty three infants (6.6%) had a birth weight < 10 th centile when matched for gestation, including three infants with a birth weight < 2.5 kg. The mean error in estimated fetal weight in these infants was 6.3%. Forty seven infants (13.4%) had a birth weight > 90 th centile. The mean error in estimated fetal weight in this group was −4.7%. Conclusions: Ultrasound determined estimated fetal weight performed just prior to labour shows a strong correlation with birth weight. In this study we observed an overestimation of birth weight in small for gestational age infants, and an underestimation of birth weight in large for gestational age infants. The incidence of undiagnosed small for gestational age neonates in this study was greater than 1:20, confirming the difficulty in identifying these babies by clinical assessment alone.