At the end of 2006, world population was approximately 6.6 billion persons (US Census Bureau, 2006). Currently the annual rate of growth of world population is about 1.3 percent, considerably lower than the peak value of 2.1 percent in the latter half of the 1960s. Growth has slowed because of reductions in fertility, which in many countries is now below the replacement level of approximately 2.1 births per woman, as measured by the total fertility rate (TFR). Between 1970 and 1975 and 2000 and 2005, the number of countries with below-replacement fertility increased from 19 to 65. Most of these countries are in the world’s more-developed regions, but the number in less-developed regions also increased, from 0 to 19. Worldwide over this same period, the number of countries with “lowest-low” fertility (TFR below 1.3) increased from 0 to 17.