CHARTS OF FETAL SIZE .4. FEMUR LENGTH

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.