In this paper, we analyse the causes of under ?ve mortality in Zambia, with a particular emphasis on assessing possible time-variations in the effects of covariates, i.e. whether the e?ects of certain covariates vary with the age of the child. The analysis is based on micro data from the 1992 Demographic and health Survey. Employing a Bayesian dynamic logit model for discrete time survival data and Markov-Chain Monte Carlo methods, we ?nd that there are several variables, including the age of the mother and the breastfeeding duration whose e?ects exhibit distinct age-dependencies.In the case of breastfeeding, this age dependency is intimately linked with the reasons for stopping breastfeeding. Incorporating such age dependencies greatly improves the explanatory power of the model and fields new insights on the di?erential role of covariates on child survival.