This study provides a unified growth theory to correctly predict the initially negative and subsequently positive relationship between child mortality and net reproduction observed in industrialized countries over the course of their demographic transitions. The model captures the intricate interplay between technological progress, mortality, fertility and economic growth in the transition from Malthusian stagnation to modern growth. It identifies a number of structural breaks over the course of development, suggesting a high degree of complexity regarding the relationships between various economic and demographic variables.