Poor maternal and newborn health and nutrition remain significant contributors to the burden of disease and mortality. Calcium supplementation has the potential to reduce adverse gestational outcomes, in particular by decreasing the risk of developing hypertensive disorders during pregnancy, which are associated with a significant number of maternal deaths and considerable risk of preterm birth, the leading cause of early neonatal and infant mortality. Member States have requested guidance from the World Health Organization (WHO) on the efficacy and safety of calcium supplementation in pregnant women as a public health strategy, in support of their efforts to achieve the Millennium Development Goals and the global targets set in the maternal, infant and child nutrition comprehensive implementation plan. The guideline is intended for a wide audience including policy-makers, their expert advisers, and technical and programme staff at organizations involved in the design, implementation and scaling-up of nutrition actions for public health