Stable isotope analysis of bone collagen is frequently employed as a means of studying the breastfeeding and weaning practices (BWP) of archaeological populations. Such studies are strengthened greatly through the application of statistical models that permit precise and model-bound estimates of weaning age, duration, trophic enrichment, and the isotopic characterization of supplementary foods. Here we present the result of a stable isotope (N-15) and Bayesian computational modeling study of bone collagen from human subadults from two coastal cemetery sites located near the mouth of the River Loa in the Atacama Desert. Recent bioarchaeological and paleodemographic research on remains from these marine hunter-gatherer sites, which are contemporary with the Formative Period (1500 BC-AD 400), has found evidence for notably elevated rates of female fertility. Ultimately, we argue that the modeled BWP parameters, which indicate the early introduction of supplementary foods, support an argument of high fertility as gleaned from the bioarchaeological evidence, and that these results provide novel insights into the child-rearing practices of the coastal populations of the Atacama. Indeed, these populations would have seemed to have developed a set of BWP that carefully balanced the biological and economic production/reproduction of the community.