Ultrasonic fetal abdominal circumference: comparison of direct versus calculated measurement

The ultrasonically derived fetal abdominal circumference is one of the essential parameters used to predict birth weight, assess fetal growth, and follow the evolution of fetal ascites. Growth curves relating abdominal circumference to gestational age have been reported for directly measured abdominal circumferences. However, none have been reported for abdominal circumferences calculated from measurements of fetal abdominal diameters. In this report, 197 normal fetuses ranging from 18 to 41 weeks' gestation are studied. The validity of 468 abdominal circumferences calculated from fetal abdominal diameters is tested by comparing the data with a similar number of direct abdominal circumference measurements obtained by a digitizer. From 18 to 41 weeks' gestation, directly measured fetal abdominal circumferences are significantly larger than fetal abdominal circumferences calculated from abdominal diameters (P less than .0001). The authors conclude that clinical management decisions based on abdominal circumference data are predicated on the use of appropriate abdominal circumference and/or abdominal diameter growth curves.