Rates of childhood obesity have increased dramatically in the last decades but little is known about the origins of, or persistence in, childhood obesity. In this paper, we attempt to answer three questions. First, how does weight status evolve from birth through primary school? Second, what is the causal effect of past weight status on the future weight status of children? Third, how important are time-varying and time-invariant factors in the dynamics of childhood obesity? We find that weight status is highly persistent from infancy through primary school and most of this persistence is driven by time-invariant factors that are determined prior to birth. Future research is needed to identify these factors.