Low birth weight is associated with increased risk of dying in the first year of life. This study was motivated by recent changes in the determination of birth weight patterns with the advent of the perinatal epidemiological transition. We analyzed data from the Brazilian National Survey of Demographic and Health of Children and Women including only children < 24 months. Prevalence of low birth weight in Brazil was 6.1%. Risk factors included female gender, residence in the South and Southeast geographic regions, low maternal education, and maternal smoking. The low birth weight profile changed, with higher prevalence in more economically developed regions, reflecting the neonatal epidemiological transition determined by changes in patterns of childbirth care and incorporation of perinatal life support technologies, in addition to the previously known biological risks associated with poverty and misinformation.