CHARTS OF FETAL SIZE .2. HEAD MEASUREMENTS

Objective To construct new size charts for fetal head circumference, b iparietal diameter and other head dimensions. Design A prospective, cr oss sectional study. Setting The routine ultrasound department of a Lo ndon 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 eac h gestational age. Centiles were derived by combining these two regres sion models, assuming that the measurements have a normal distribution at each gestational age. Results A total of 594 fetuses had their bip arietal diameter measured and their head circumference measured direct ly. Both head diameters were recorded for 587 fetuses and the circumfe rence was also derived from these, as was the cephalic area. New chart s are presented for biparietal diameter (both outer-outer and outer-in ner), head circumference (directly measured and derived from diameters ). The directly measured head circumferences were consistently (by abo ut 1%) greater than those derived from measurement of the head diamete rs. The new charts are compared with previously published charts that are in wide use. Charts for occipitofrontal diameter, cephalic index a nd cephalic area are also presented. Conclusions We have constructed n ew size charts for the fetal biparietal diameter and for head circumfe rence, 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 appropr iately derived chart when assessing the head circumference. The differ ences between the new charts for biparietal diameter and head circumfe rence and previous ones may be largely due to methodological differenc es.