During pregnancy, the metabolic requirements of the mother are increased; however, the relationship between maternal intake of key nutrients and optimal fetal growth is not always clear. In this chapter, we have reviewed randomized controlled trials of nutritional interventions during pregnancy, with a particular focus on birthweight and infants who are small for gestational age (SGA). Of the trials that have investigated changing macronutrient and energy intakes during pregnancy, supplements in which <25% of the energy is provided by protein yielded the most promising results, producing a 31-32% reduction in the risk of SGA infants and an increase in birthweight (38-60 g) compared with control. Single-nutrient intervention trials using n-3 long-chain polyunsaturated fatty acid (LCPUFA) supplements demonstrated small increases in birthweight (≈50 g) and birth length (≈0.5 cm), which may be explained by small increases in gestation length (approximately 2.5 days). n-3 LCPUFA supplementation in pregnancy did not however decrease the proportion of SGA infants. Multiple-micronutrient supplementation trials in developing countries have resulted in increased mean birthweight (22-44 g) and reduced the risk SGA by 9-15%. Further nutritional intervention studies which are rigorously designed and implemented are needed particularly to delineate differential effects in developed and developing countries.