To provide standards for evaluating fetal thigh circumference measurements obtained with ultrasound, a study of the growth of this parameter during pregnancy was made. Because of possible sampling problems before 22 weeks (menstrual age), this investigation was limited to the 22-40 week time period. Regression analysis indicated that the optimal model (R2 = 88.9%) for describing thigh circumference (ThC) growth was a power function. ThC variability increased with menstrual age, from 1.3 cm at 22 weeks to 3.1 cm at 40 weeks. These data were used to construct a standard growth curve for the ThC.