Predicted rates of period change for the first crossing of the instability strip as a function of stellar radius for the computed grid of stellar evolution models assuming a Cepheid mass-loss rate of 10^-6 M_⊙ yr^-1. The assumed mass-loss rate has negligible effect on the period change during the first crossing, hence the comparison using grids with other mass-loss rates has the same result. The grey box represents the measured rate of period change and radius if the distance is 99 pc.