Charts of fetal size: 3. Abdominal measurements

OBJECTIVE: To construct new size charts for fetal abdominal circumference and area. DESIGN: A prospective, cross sectional study. SETTING: The routine ultrasound department of a London teaching hospital. SUBJECTS: The fetuses of 663 women seen in the routine antenatal booking clinic whose ultrasound and menstrual dates agreed within 10 days. METHODS: Fetuses were scanned once only for the purpose of the study at gestations between 12 and 42 weeks, when up to 20 dimensions were measured. Separate regression models were fitted to estimate the mean and standard deviation as functions of gestational age. Centiles were derived by combining these two regression models, assuming that the measurements have a Normal distribution at each gestation. RESULTS: A total of 610 fetuses had their abdominal circumference measured directly. Abdominal diameters were recorded for 425 fetuses and the circumference was also derived from these, as was the abdominal area. New charts for abdominal circumference (directly measured and derived from diameters) are presented. The directly measured circumferences were consistently (by about 3.5%) greater than those derived from measurement of the abdominal diameters. The new charts are compared with previously published charts that are in wide use. A chart for abdominal area is also presented. CONCLUSIONS: We have constructed new size charts for the fetal abdominal circumference, both measured directly and derived from abdominal diameters. We have demonstrated the difference between the size charts constructed from these two sets of values and hence the importance of using the appropriately derived chart when assessing the abdominal circumference. The differences between the new charts and previous ones may be largely due to methodological differences.