A low prepregnancy body mass index is one of strongest predictors of adverse pregnancy outcomes such as preterm birth and fetal growth retardation. A low body mass interacts with other risk factors such as smoking and stress to increase risk of these outcomes, whereas zinc supplementation and low-dose aspirin increase birth weight in thin but not normal-size women. The association between maternal thinness and adverse pregnancy outcomes may be mediated more by a low plasma volume than by decreased protein or energy status. Maternal micronutrient status may partially mediate plasma volume expansion in pregnancy. Therefore, improving maternal micronutrient status may reduce adverse outcomes through this mechanism.