BackgroundThere is a limited understanding of the dynamic influences that shape infant and young child feeding (IYCF) decisions over time. We conducted an innovative qualitative study to reconstruct IYCF trajectories across early life course phases, in the context of the socioecological model (SEM) and the commercial determinants of IYCF.MethodsWomen of different socioeconomic status were interviewed in two large metropolitan areas in Mexico. Our specific goal was to allow us to better understand if and how the commercial milk formula (CMF) marketing influenced breastfeeding decisions in a complex dynamic way involving the individual, relational, community and societal levels.ResultsHospitals, health professionals, and interactions with social media were key category entry points throughout the prenatal, perinatal, early infancy period and beyond. The CMF industry interfered by engaging a wide array of actors across the different layers of the SEM, most prominently the health care system and the workplace. Through its marketing strategies the CMF operates subconsciously and its messages are most effective when health institutions, health care providers, workplace spaces and social norms are weak in their support for breastfeeding.ConclusionsThe cases in our study highlight how, together with a weak breastfeeding counseling system, and health professionals who lack training in breastfeeding and normal infant behavior, lead to the opportunity for CMF marketing to shape infant feeding, and ultimately to the decision to feed formulas that some mothers were not planning to use and cannot afford.