The relation between fetal head circumference and menstrual age was determined by cross-sectional analysis of 400 fetuses (15-41 weeks) examined with a linear array real-time (dynamic image) scanner using specifically defined methodology. Mathematical modeling of the data demonstrated that the head circumference growth curve is nonlinear, similar to the biparietal diameter growth curve. Both the linear quadratic (r2 = 98.3%) and linear cubic (r2 = 98.3%) functions could be considered the optimal model. Predicted head circumference values for the linear cubic function were comparable to established normal values on the basis of postnatal measurements at 25-40 weeks (mean difference, -0.94 mm; SD, 0.47 mm). Predicted menstrual age values (+/- 2 SD) associated with a given head circumference measurement were calculated and are presented in tabular form. The use of the head circumference measurement in predicting menstrual age, estimating fetal weight, and in detecting intrauterine growth retardation is discussed.