Objective To construct a new size chart for fetal femur length. Design
A prospective, cross sectional study of fetuses scanned once only for
the purpose of the study at gestations between 12 and 42 weeks. Setti
ng The routine ultrasound department of a London teaching hospital. Su
bjects The fetuses of 663 women seen in the routine antenatal booking
clinic whose ultrasound and menstrual dates agreed within 10 days. Res
ults Femur length was measured on 649 of the 663 fetuses. A linear-cub
ic regression model was fitted to estimate the mean and a separate lin
ear regression to estimate the standard deviation. Centiles were deriv
ed by combining these two regression models, assuming that the measure
ments have a normal distribution at each gestational age. A new chart
for femur size is presented and compared with previously published dat
a. Conclusions We have constructed a new size chart for fetal femur le
ngth taking into consideration the increasing variability with increas
ing gestational age. We have compared our chart with other published d
ata, and believe that the differences seen may be largely due to metho
dological differences.