The relationship between maternal hematocrit and pregnancy outcome at various times in pregnancy was studied in 17,149 iron- and folate-supplemented pregnant women. On univariate analysis, early-pregnancy hematocrits below 37% were associated with preterm delivery. However, this relationship was not confirmed by multivariate analysis controlling for other risk factors. On both univariate and multivariate analyses, both early and later in pregnancy, hematocrits above 40% were associated with preterm delivery. In every gestational time period, at least part of the excess of preterm births was explained by an increase in indicated preterm deliveries. In both early and late pregnancy, and in both the univariate and multivariate analyses, only high hematocrits were associated with fetal growth retardation. The strongest association (odds ratio above 2) between high hematocrit and both fetal growth retardation and preterm delivery occurred with hematocrits at or above 43% at 31–34 weeks’ gestation. © 1991 The American College of Obstetricians and Gynecologists.