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.