Modelling the Inverse Association between Breastfeeding and Contraceptive Use

Previous studies of the inverse association between breastfeeding and contraceptive use have relied on current status data on at least one of the variables. In this paper we suggest a new way of modelling the joint temporal association of the adoption of contraception and the termination of breastfeeding over the course of a birth interval. The approach is implemented with survey data from rural Mexico. It involves first estimating a competing-risks model in which both hazards are considered simultaneous, together with a set of relevant covariates. Conditional analyses are then carried out to estimate the hazards of each event conditional on the timing of the other. By comparing the covariates, we gauge the extent to which the occurrence of one event raises or lowers the hazard of the other. The results show that, in rural Mexico, the timing of the termination of breastfeeding and that of starting contraceptive use are closely related, and that there exists a surprisingly neat interval of 120 days in which the action takes place.