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. 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. RESULTS: Femur length was measured on 649 of the 663 fetuses. A linear-cubic regression model was fitted to estimate the mean and a separate linear regression to estimate the standard deviation. Centiles were derived by combining these two regression models, assuming that the measurements have a normal distribution at each gestational age. A new chart for femur size is presented and compared with previously published data. CONCLUSIONS: We have constructed a new size chart for fetal femur length taking into consideration the increasing variability with increasing gestational age. We have compared our chart with other published data, and believe that the differences seen may be largely due to methodological differences.