Vitamin D deficiency is thought to be common among pregnant women, particularly during the winter months, and has been found to be associated with an increased risk of pre-eclampsia, gestational diabetes mellitus, preterm birth, and other tissue-specific conditions. Recent scientific literature has reported the effects of vitamin D supplementation on adverse maternal and infant outcomes. Member States have requested guidance from the World Health Organization (WHO) on the effects and safety of vitamin D 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. This 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.