Charts of fetal size: 2. Head measurements*

OBJECTIVE: To construct new size charts for fetal head circumference, biparietal diameter and other head dimensions. 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. For each measurement separate regression models were fitted to estimate the mean and standard deviation at each gestational age. Centiles were derived by combining these two regression models, assuming that the measurements have a normal distribution at each gestational age. RESULTS: A total of 594 fetuses had their biparietal diameter measured and their head circumference measured directly. Both head diameters were recorded for 587 fetuses and the circumference was also derived from these, as was the cephalic area. New charts are presented for biparietal diameter (both outer-outer and outer-inner), head circumference (directly measured and derived from diameters). The directly measured head circumferences were consistently (by about 1%) greater than those derived from measurement of the head diameters. The new charts are compared with previously published charts that are in wide use. Charts for occipitofrontal diameter, cephalic index and cephalic area are also presented. CONCLUSIONS: We have constructed new size charts for the fetal biparietal diameter and for head circumference, both measured directly and derived from head 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 head circumference. The differences between the new charts for biparietal diameter and head circumference and previous ones may be largely due to methodological differences.