If the pre–World War II period was one in which major changes in mortalityand morbidity were most significant, the major theme in the immediate postwarperiod was the abrupt changes in fertility and migrations that occurred in thisperiod of transition. There were, of course, continued declines in the rates ofmorbidity and mortality, but these were now more evenly spaced among all agegroups owing to the massive introduction of antibiotics. But it was changes infertility that had the most significant impact on the demographic structure ofthe nation. In what would prove to be a temporary change in direction withimportant long-term implications, the national population reversed itscentury-and-a-half-long secular decline in fertility and began moving towardhigher birth rates. By the end of this period, that trend would be reversed andreplaced by startling new trends in both fertility and family organization. Atthe beginning of this period, foreign immigration reached its lowest point inover a century and was replaced by significant African American migration fromthe South to the urban North and by the migration of island-born Puerto Ricansto the mainland United States. By the end of this era, however, internationalimmigration had completely reversed its trend and a whole new chapter in thehistory of immigration to the United States began with the participation of newpeoples populating the nation. This postwar period was also a time when internalmigrations of native-born Americans began to define some new and rather uniquelyAmerican patterns in urban settlement, with the rise of the suburbs and thecorresponding changes in the inner cities. Finally, this was a period of themost intense African American migration, when the then-largest minoritypopulation in the national population ended its massive migration out of theSouth.