The reproductive cycle refers to the process from a woman's first to last birth. Through a systematic categorization and analysis of the evolution of reproductive cycles across global regions, combined with theoretical discussions on long-term demographic development, this study reveals that reproductive cycle evolution exhibits significant ergodicity, termed the Reproductive Cycle Transition (RCT). The RCT progresses through four stages: a Steady-State Stage, a Compression Stage, a Leveling-Off Stage, and an Equilibrium Stage. It shifts from a traditional pattern characterized by a low age at first birth, a high age at last birth, and a long reproductive span (“low-high-high”) to a modern pattern characterized by a high age at first birth, a low age at last birth, and a short reproductive span (“high-low-low”). The RCT parallels the fertility transition. Europe, North America, Australia, New Zealand and East Asia have entered the Leveling-Off Stage. In contrast, most of Asia, Africa, and Latin America are in the late stage of the Compression Stage, while East, Central, and West Africa remain in their early stage. China's reproductive cycle exhibits an accelerated pattern, beginning in the 1960s and entering the Leveling-Off Stage by the early 1990s. The shortening of the reproductive span carries the risk of lowest-low fertility. Fertility support policies should target the distinct fertility needs of women across different age groups.

By combining the places of residence at present, five years ago and at birth, and the place of household registration from census tabulations, migrations in China can be divided into basic categories including long-term migration, short-term migration, direct migration, nominal migration and circulation. Based on both macro and micro data from 2020 census, the scale and intensity of various types of inter-provincial migration are estimated, revealing differences in cultural stickiness and institutional resistance at both origin and destination. To avoid ambiguity, it is recommended that “Inter-X Migration” stands for the migration crossing the area boundaries of level X but not crossing the boundaries of upper level, “Intra-X Migration” stands for the migration crossing the area boundaries below level X, “Cross-X Migration”stands for the migration crossing the area boundaries of level X and above. The Courgeau model indicates that the Crude Migration Intensity (CMI) for lifelong migration in China is 49.5%, while the CMI for five-year migration is 21.0%, showing that Chinese people are becoming more migratory as urbanization progresses. The Rogers age patterns of migration also correspond to contemporary China with some peculiarities in the middle and older age peak. The high proportion of adolescents in nominal migration means that the household registration system still prevents the descendants of migrants from settling down de jure.

A three-group age structure can be designated as a point in the triangular diagram, while the routine of the points in time series traces the age structure evolution. The paper classifies the age structure evolution patterns of countries (areas) in the world and provinces in China, and discovers the general and ergodic age structure transition (AST) pattern which experiences three stages: the “bonus raising” stage (i.e., from the traditional fertile young population to fewer children-more labors population), the “debted ageing” stage (i.e., from fewer children-more labors population to fewer labors-more aged population), and the stage in between (i.e., turning stage). While developed countries are primarily in the middle of stage three, most developing countries are in the late phase of the first stage, and many least developed countries in Africa are still in the early stage one or even in the traditional position. Started in the 1950s and accelerated since the 1980s, the AST in China is now just passing the turning stage with diversifying decreasing bonuses at turning points from coast to inland provinces. According to a simulation scenario, China will approach the equilibrium position near 2100, with 15% children, 55% labor forces, and 30% older people, which is probably the common end of all countries in the future.

This paper constructs an explanatory model of fertility changes based on social dynamics, which consists of three factors affecting human reproductive behavior, including fertility intentions, contraceptive barriers and competing preferences. The changes in the relative effects among these three factors contribute to three stages of fertility change in human history: natural fertility, shrinking fertility and suppressed fertility. In China, family planning policy is the fourth factor. With the promotion, implementation and relaxation of the family planning policy, Chinese reproductive behaviors have adjusted accordingly, leading to fluctuating fertility rates. The building-up of market economy in early 1990s stimulated the competing preferences. Based on data for East China from the 2017 fertility survey, this paper examines trends in total fertility rate, marital duration-specific fertility rate and parity-specific total fertility rate. With Shanghai in center, fertilities by provinces differ latitudinally. We also show the changes of monthly distribution of births since 2012 to see the influence of family planning policy adjustment.

China has been urbanizing and big cities have been growing rapidly.Number of megacities in China increased from 34 in 1997 to 63 in 2007.There are substantial disputes over China’s urbanization strategy to prioritize big cities or medium and small-sized cities.Development of big cities faces challenges in transportation,housing,environment and resources and public services,and population control becomes increasingly important in big cities.This issue of Population and Development Forum is specifically devoted to population control in China’s megacities,for which three population experts were invited to share their views and suggestions.Professor Huang Ronglong,from Nanjing Normal University,argues that the major cause of China’s urban disease is not overpopulation but ill management of the cities.However,Professor Ding Jinhong,from East China Normal University,has a different view and asserts that a permanent cure to urban disease is population control.Talking on population control in Beijing,Professor Duan Chengrong,from People’s University of China,suggests the population control strategy be changed from a local to a national perspective.Three experts also provide policy suggestions in population control,city planning and management,industrial upgrading,mechanisms in evaluating demographic effect of social and economic programs,etc.Professor Liu Feng,from Development Research Center of the State Council,chairs the Forum and provides comments to the three papers and his own opinions as well.