PIP: Gompertz's procedure of linearized function through logarithmic transformation, based on Brass' method, is used in this study to make adjustments of fertility structure, and, consequently, to evaluate criteria to project it. This procedure is applied to data gathered in Chile and in Costa Rica. While up to now studies paid attention to the estimation of fertility level and relative age distribution, this procedure will make possible the study of fertility age structure as far as behavior, correction of its changes, and probable trends are concerned. The procedure, which is thought to be very flexible, is described in detail in this highly technical article.