Article dans une revue: Parenthood postponement is a key demographic trend of the last three decades. In order to rationalize that stylized fact, we extend the canonical model by Barro and Becker (Econometrica 57:481–501, 1989) to include two—instead of one—reproduction periods. We examine how the cost structure of early and late children in terms of time and goods affects the optimal fertility timing. Then, we identify conditions that guarantee the existence and uniqueness of a stationary equilibrium with a stationary cohort size. Finally, we examine how the model can rationalize the observed postponement of births, and we highlight two plausible causes: (1) a general rise in the cost of children in terms of goods and (2) a decline in the degree of family altruism.