ULTRASONIC MEASUREMENT OF FETAL ABDOMEN CIRCUMFERENCE IN THE ESTIMATION OF FETAL WEIGHT

TLDR

The study presents a method to estimate fetal weight using ultrasound measurement of abdominal circumference. In 140 fetuses, the method’s accuracy varied with size: at 1 kg, 95 % of birth weights were within 160 g; at 2, 3, and 4 kg, the limits were 290 g, 450 g, and 590 g, respectively, with constant percentage confidence limits; extrapolation indicates that a single 32‑week measurement would detect 87 % of babies below the 5th centile, dropping to 63 % at 38 weeks, while the false‑positive rate stays just over 1 %.

Abstract

Summary A method of estimating fetal weight by ultrasonic measurement of the fetal abdominal circumference is described. Assessment of birth weight predictions on 140 fetuses who were delivered within 48 hours of this measurement showed that the accuracy of predictions varied with the size of the fetus; at a predicted weight of 1 kg, 95 per cent of birth weights fell within 160 g, while at 2 kg, 3 kg and 4 kg the corresponding values were 290 g, 450 g and 590 g respectively. Expressed as a percentage of the predicted weight, confidence limits remained constant throughout the birth weight range. Extrapolation of these data to routine screening of the obstetric population showed that with a single measurement at 32 weeks menstrual age, 87 per cent of babies below the 5th centile would be detected by this method but that the diagnosis rate would fall to 63 per cent at 38 weeks. The false positive diagnosis rate would remain constant between 32 and 38 weeks at just over 1 per cent.