1 Revisiting mid-twentieth century fertility shifts from a global perspective David Reher (Universidad Complutense de Madrid y GEPS) dreher@geps.es Miguel Requena (UNED y GEPS) mrequena@poli.uned.es Abstract The historic process of fertility decline was interrupted during the central decades of the twentieth century with an unexpected period of increasing fertility that has been called the baby boom. This boom has received widespread attention in the developed world where it is a mainstay in the existing literature. Normally it is considered a phenomenon exclusive to countries participating in the historic demographic transition. Recent studies suggest that a similar trend change in fertility may have also taken place in some developing nations at approximately the same time and with similar characteristics to the fertility boom in the developed world. The main goal of this paper is to examine the extent to which these trend changes took place in an important sample of developing countries, examining cohort data from a comparative perspective. Data for this analysis are taken from the Integrated Public Use Microdata Series, International (IPUMS-I). The analysis shows that the shift upwards of fertility for cohorts born during the 1930s appears to be a trend common to many countries in both the developed and the developing world. Introduction In many developed nations, the historic process of demographic transition was interrupted during the central decades of the twentieth century by a period of unexpected fertility increase that has been called the baby boom. During this period, reproduction dynamics shifted with an acceleration of nuptiality together with more or less important increases in total fertility leading to a generalized increase in yearly totals of births. The process itself was diverse in timing and intensity, with substantial fertility increases in some societies and very modest