This article is devoted to the study of the Pareto-efficiency of the competitive equilibrium for a simple overlapping generations economy with endogenous fertility. For CES utility and production functions, it is proved that the economic properties of the economy are closely related to the two elasticities
of substitution. First, the competitive equilibrium exists and is unique if the sum of the two elasticities is not smaller than one. Secondly, a set of parameters is provided such that the equilibrium is both in under-accumulation and inefficient. Thirdly, a sufficient condition is proved that ensures that an equilibrium converging in under-accumulation is Pareto-efficient: the sum of the two elasticities must not be greater than two.