The relationship between growth of the fetal femur and foot length was examined between 14-40 weeks' gestation in 182 normal singleton pregnancies. A significant correlation was demonstrated (r = 0.98; P less than .0001). The femur/foot length ratio was found to be approximately 1 throughout this age range. The fifth and 95th percentile confidence intervals of the femur/foot length ratio versus gestational age were then defined. The femur/foot length ratio was also measured directly in 12 fetuses after second-trimester abortion; these measurements compared favorably with the ultrasound femur/foot length ratio nomogram. The discriminatory power of the femur/foot length ratio chart to differentiate fetuses with skeletal dysplasias from those with pseudo-limb reduction due to constitutional factors or intrauterine growth retardation (IUGR) was tested in 16 sequential cases with femur measurements below the fifth percentile. Two cases suspected of having symmetrical IUGR and five cases suspected of being constitutionally small were found to have femur/foot length ratios within the 90th percentile range. Limb reduction deformities were suspected in the nine remaining cases, and the femur/foot length ratios in these fetuses were all found to be less than the fifth percentile. The femur/foot length ratio nomogram appears to be a useful parameter to help differentiate fetuses that have dysplastic limb reduction from those whose limbs are short because of constitutional factors or IUGR.