The risk of developing many adult onset diseases, including hypertension, type II diabetes and renal disease is increased in individuals born of a low birth weight. A potential underlying mechanism contributing to the onset of these diseases is the formation of a low nephron endowment during development. Evidence from the human, as well as many experimental animal models, has found a strong association between being born of a low weight and having a reduced nephron endowment. However, other animal models, particularly those in which the mother is exposed to elevated glucocorticoids for a short period, have shown a 20-40% reduction in nephron endowment without discernible changes in the birth weight of offspring. This highlights that a low birth weight is one but certainly not the only predictor of nephron endowment and suggests that even individuals born of a normal birth weight could potentially have a reduced nephron endowment and be at risk of developing adult onset disease. Recognition of the dissociation between birth-weight and nephron endowment is important for future studies aimed at elucidating the role of a reduced nephron endowment in the developmental programming of adult disease.