The purpose of this study was to describe the ultrasonic measurement of fetal foot length and to develop mathematical models to quantify the relationships between menstrual age and commonly measured fetal structures. We evaluated 120 patients with known last menstrual periods and normal pregnancies to develop a cross-sectional study population. All patients had ultrasonic measurement of the fetal foot length, biparietal diameter, head circumference, abdominal circumference, and femur length. Least-squares estimation of linear models was used to select the best mathematical models to describe the relationship between menstrual age and fetal foot length. A similar evaluation of the relationship between fetal foot length and the other measured parameters was performed. All models were best described by a linear equation. An R2 value of 0.94, with a standard error of the estimate of 0.204, was obtained for menstrual age versus fetal foot length. When the model for fetal foot length and menstrual age was compared with published anatomical data, close agreement was seen over the time interval studied. Our results suggest that the measurement of fetal foot length with ultrasound gives a reliable assessment of anatomical fetal or neonatal foot length and is highly correlated to the menstrual age of the fetus.