When maternal anemia is diagnosed before midpregnancy, it has been associated with an increased risk of preterm delivery. Maternal anemia detected during the later stages of pregnancy, especially the third trimester, often reflects the expected (and necessary) expansion of maternal plasma volume. Third-trimester anemia usually is not associated with increased risk of preterm delivery. High hemoglobin concentration, elevated hematocrit and increased levels of serum ferritin late in pregnancy, however, all have been associated with increased preterm delivery. This increased risk may reflect in part the failure to expand maternal plasma volume adequately, thus diminishing appropriate placental perfusion. Although controlled trials of iron supplementation during pregnancy have consistently demonstrated positive effects on maternal iron status at delivery, they have not demonstrated reductions in factors that are associated with maternal anemia, i.e., increased risk of preterm delivery and infant low birth weight. One reason for discordant findings may be the exclusion of many gravidas with iron deficiency from these trials or the data concerning gravidas with pregnancy outcomes such as preterm delivery from the analysis. Finally, recent concerns have been voiced about harmful effects of iron supplementation during pregnancy. No adverse effects of iron supplementation on pregnancy outcome have been demonstrated to date. Questions about the efficacy of iron supplementation during pregnancy for reducing adverse outcomes such as preterm delivery and side effects from iron supplementation, including the potential for oxidation of lipids and DNA, require further research in iron-deficient women.