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This article presents a modification of the growth function of Bridges et al. (1986) that incorporates a known biological time unit (gestation period) to represent physiological time for a given animal species. Gestation time was incorporated into the weight-age equation and modified by the growth rate time constant, c, which relates the inflection point of the growth curve to a relatively fixed time unit. Relating the inflection point of the growth curve to an animals gestation period provides a mathematical basis for changing growth curves to account for the external influences of management, diet, environment and health that may impact an animals growth in approaching mature size. Examples are shown using the developed mathematical expression to predict changes in the growth of two different cattle under the effects of early weaning and different nutritional and environmental conditions.