The problem of the future viability of pension systems is a hot topic in the OECD countries. Demographic changes, which include a low birth rate and an ageing population, are directly related to the pension system crisis. Hence the need to study the fertility rate and the factors involved before implementing any possible reform in pension plans. This paper presents two models: one which analyzes the economic burden of having children and another which looks at the cost in terms of time. It concludes that it is reasonable to link pensions to the number of children.