Gestational weight gain is positively associated with fetal growth, and observational studies of food supplementation in pregnancy have reported increases in gestational weight gain and fetal growth.To assess the effects of advice to increase or reduce energy or protein intake, or of actual energy or protein supplementation or restriction, during pregnancy on energy and protein intakes, gestational weight gain, and the outcome of pregnancy.We searched the Cochrane Pregnancy and Childbirth Group trials register (October 2002) and contacted researchers in the field.Acceptably controlled trials of dietary advice to increase or reduce energy or protein intake, or of actual energy or protein supplementation or restriction, during pregnancy.Data were extracted by the authors from published reports, and supplemented by additional information from trialists contacted by the authors.In five trials involving 1134 women, nutritional advice to increase energy and protein intakes was successful in achieving those goals, but no consistent benefit was observed on pregnancy outcomes. In 13 trials involving 4665 women, balanced energy/protein supplementation was associated with modest increases in maternal weight gain and in mean birth weight, and a substantial reduction in risk of small-for-gestational-age (SGA) birth. These effects did not appear greater in undernourished women. No significant effects were detected on preterm birth, but significantly reduced risks were observed for stillbirth and neonatal death. In two trials involving 1076 women, high-protein supplementation was associated with a small, nonsignificant increase in maternal weight gain but a nonsignificant reduction in mean birthweight, a significantly increased risk of SGA birth, and a nonsignificantly increased risk of neonatal death. In 3 trials involving 966 women, isocaloric protein supplementation was also associated with an increased risk of SGA birth. In three trials involving 384 women, energy/protein restriction of pregnant women who were overweight or exhibited high weight gain significantly reduced weekly maternal weight gain and mean birth weight but had no effect on pregnancy-induced hypertension or pre-eclampsia.Dietary advice appears effective in increasing pregnant women's energy and protein intakes but is unlikely to confer major benefits on infant or maternal health. Balanced energy/protein supplementation improves fetal growth and may reduce the risk of fetal and neonatal death. High-protein or balanced protein supplementation alone is not beneficial and may be harmful to the infant.Protein/energy restriction of pregnant women who are overweight or exhibit high weight gain is unlikely to be beneficial and may be harmful to the infant.