Births averted by contraception: an approach through renewal theory.

Of critical interest to family planning administrators are estimates of births averted per segment of contraception. By a “segment” of contraception is meant the period of usage of a method between its initiation (or resumption after a period of nonuse) and its next interruption owing to accidental pregnancy, switch to another method, or stopping contraception. Several attempts to estimate births averted per IUD inserted have been pub1ished.l These analyses have been too tailored to specific data and too empirical to provide an ideal vehicle for clarifying the conditions under which births averted per segment of contraception is high or low. Nevertheless, these applied analyses have served to establish some of the factors that deserve place in a theory of family planning impact. Because they provide such efficient protection against pregnancy, modern contraceptives such as the IUD or the antiovular pills have an impact that is limited less by accidental pregnancy than by high discontinuation rates for other reasons. Another qualifying factor is the amount of overlap between practice of the contraceptive and the anovulatory period following childbirth when the woman is protected anyway. Even when all these factors are held