Pre-print, Working paper: During the last two centuries, fertility has exhibited, in industrialized economies, two distinct trends: the cohort total fertility rate follows a decreasing pattern, while the cohort average age at motherhood exhibits a U-shaped pattern. This paper proposes a unified growth theory aimed at rationalizing those two demographic stylized facts. We develop a three-period OLG model with two periods of fertility, and show how a traditional economy, where individuals do not invest in higher education, and where income rises push towards advancing births, can progressively converge towards a modern economy, where individuals invest in higher education, and where income rises encourage postponing births. Our findings are illustrated numerically by replicating the dynamics of the quantum and the tempo of births for Swedish cohorts born between 1876 and 1966.