Pre-print, Working paper: During the last century, 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 education, and where income rises push towards advancing births, can progressively converge towards a modern economy, where individuals invest in 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 cohorts 1906-1975 of the Human Fertility Database.