A synthetic cohort of women is followed from birth through childbearing age. The transition from parity i to parity i + 1 is quantitatively described utilizing systems analysis theory. This description results in a parametrization of the renewal process in which the net maternity function is completely determined by five parameters: maturation time, 0th parity time constant, survival fraction to 0th parity, average child spacing time, and parity progression ratio. To detect trends in these parameters, estimates are made of them for the U. S. A. population from 1950 to 1965.

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