Conceptive delay, nonsusceptible period, and cohort fertility patterns.

A model of cohort fertility pattern by duration of marriage is built on the parameters of conceptive risk, its decline along parity, and the length of the nonsusceptible period. The model is found to fit satisfactorily a variety of cohort fertility patterns. Birth intervals by parity are then inferred from the estimated parameters. The present model is developed for comparative analyses of fertility trends and for fertility projections which take into account ongoing changes in birth intervals and completed family-size distributions which may be observed in current sample surveys.