Changes in Birth Rates as a Function of Contraceptive Effectiveness: Some Applications of a Stochastic Model

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Abstract
The investigation is based on a model of human reproduction which includes these parameters: fecundability (the monthly probability that a susceptible woman will conceive), the probability that a conception results in a live birth, and the varying periods of temporary nonsusceptibility which follow on a conception. A contraceptive method which halved the chances of conception in any 1 month would reduce births by only about 1/5 in peoples with high fertility. In order to halve births among users, the method would have to reduce the monthyl chances of conception by more than 80%. Should only 60% of eligible persons adopt it, a method of 95% effectiveness would be required to reduce births by one half. These numerical results apply to a selected set of parameters. In all cases, however, the effect of the periods of nonsusceptibility is such that a contraceptive of specified effectiveness would reduce birth rates by less than this effectiveness. Generally, more effective methods used by smaller fractions of a population would produce a greater decline in birth rates than would less effective methods used by a larger part of the population.