Contraceptive use and its impact on fertility.

Some factors which determine the impact of a particular contraceptive on the fertility of a given population are discussed: 1) duration of use of the contraceptive, 2) proportion of the population using the contraceptive, 3) distribution of fecundability among contracepting couples (a homogeneous or heterogeneous distribution), and 4) risk of exposure to pregnancy at the time contraception is adopted. Reduction in the fertility of a population by a contraceptive of given effectiveness decreases as the duration of contraceptive use increased. A larger reduction in fertility occurs in a population in which a smaller proportion of women use a more effective contraceptive than in a group in which a larger proportion use a less effective contraceptive. Among 2 groups of women with the same mean fecundability, the reduction in fertility brought about by a contraceptive of particular effectiveness is greater in the group of women with a heterogeneous distribution of fecundability than in the group with homogeneous fecundability.