['To establish an accurate mathematical model describing fetal head growth, taking into account gender differences and changes in growth rate during gestation.', 'Ultrasound measurements of head circumference and biparietal diameter were made on 1,336 normal fetuses (684 males and 652 females) in the Maternité Régionale de Nancy (France). A new two-phase model, taking into account an alteration in growth kinetics at 30 gestational weeks, was computed independently for male and female data. The accuracy of this model was tested and compared with three current mathematical models: a linear-quadratic, a linear-cubic, and the Rossavik and Deter (1984) models.', 'In all models, including ours, the coefficients of determination (R(2)) were high (> or =0.999), so long as male and female data were computed separately. However, the standard error estimates (SEE) of our two-phase model were much lower (0.13 < or = SEE < or = 0.57) than the SEE of the three other models when computed over the whole gestational period (0.49 < or = SEE < or = 2.69); nevertheless, when these three other models were computed for these two successive periods, their SEE decreased, and data fitting was improved.'] Are only two-phase models , computed independently for males and females , appropriate to describe fetal head growth?