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[The construction of the fertility model of variable parity from family planning practice].


 Understanding the changing patterns of age specific fertility under the planning system is essential for building a fertility model which reflects birth control policy implementation in China. In building a Parity Variable Fertility Model, 4 basic elements are to be considered: 1) psychosocial, and physiological variables, 2) patterns of the total fertility rate and age-specific fertility rate, 3) socioeconomic development, and 4) distribution of parity-specific fertility. THe natural fertility of women is 17, calculated from a 309-years childbearing period, with 17% of non-susceptible time. In China, about 86% of natural fertility is suppressed by various factors. In this model, the following variables are included: 1) The first marriage ratio, which is the proportion of women in each age group which enters into a first marriage. The range and spread of this ratio is closely associated with the first birth. 2) The first birth ratio, which is the proportion of a marriage cohort to have a first birth each year. 3) the birth interval, which determines the distribution of second births. 4) Regulation coefficient B, which represents birth control regulations which approximately determine the number of second-parity or higher order births. The difference between the fertility level generated from the Parity Variable Fertility Model and reality depends on the implementation of birth control program, the assumptions on regulation coefficients, and changes in social and cultural factors. The model is easy to use, especially for areas where the marriage and fertility records of women of child-bearing are well kept.
