The Selenium-Mercury Paradigm examines the relationship between trace minerals, dietary exposure, and fetal brain development through a framework that emphasizes biological protection rather than avoidance alone. Focusing on pregnancy as a period of irreversible developmental demand, the book explains how selenium-dependent enzymatic systems regulate oxidative stress and constrain the disruptive potential of mercury when nutritional sufficiency is maintained. Drawing on chemistry, developmental biology, population data, and institutional analysis, this work addresses long-standing contradictions in public health guidance surrounding seafood consumption during pregnancy. It demonstrates why mercury-centered models have struggled to predict observed outcomes and why selenium's protective role has remained under-integrated despite extensive evidence. Written with restraint and clarity, the book does not issue dietary mandates or clinical instructions. Instead, it offers a coherent explanatory framework intended for clinicians, researchers, policy professionals, and informed readers engaged in maternal and developmental health. The emphasis throughout is on alignment-between guidance and biology, exposure and protection, risk and resilience.