We explore the optimal fertility timing in a four-period OLG economy with physical capital, whose specificity is to include not one, but two reproduction periods. It is shown that, for a given total fertility rate, the economy exhibits quite different dynamics, depending on the timing of births. If all births take place in the late reproduction period, there exists no stable stationary equilibrium and the economy exhibits cyclical dynamics due to labor growth fluctuations. We characterize the long-run social optimum and show that optimal consumptions and capital depend on the optimal cohort growth factor, so that there is no one-to-one substitutability between early and late fertility. We also extend Samuelson's Serendipity Theorem to our economy and study the robustness of our results to: (1) endogenizing fertility timing, (2) assuming rational anticipations about factor prices, (3) adding a third reproduction period.