The definition of a Low Birth Weight baby is a newborn weighing less than 2,500 g. Globally, around 15% of newborns meet this definition. The assessment of foetal growth restriction involves the comparison of the weight of the newborn with the expected weight for its gestational age. If the weight of an infant is below the 10th centile of a reference population for foetal growth, it is considered small for gestational age. The evidence clearly shows that foetal growth restriction significantly impacts on subsequent survival, health, growth and development. In this presentation, Black explains methods used to estimate the global prevalence of Small for Gestational Age (SGA). In 2010 alone, 32.4 million babies were born SGA in low- and middle-income countries. This constitutes almost a third of all live births. Given the consequences of being born SGA, data on gestational age should be used to assess risks and guide clinical care both in the pre- and postnatal settings.