Objective: The purpose of this study was to test the null hypothesis that the volume of exercise at different times during pregnancy has no effect on fetoplacental growth. Study Design: Seventy-five women who exercised regularly were evaluated before pregnancy and randomly assigned at 8 weeks' gestation to one of 3 exercise regimens for the remainder of pregnancy. Primary outcome variables included placental growth rate, birth weight, and placental volume at term. Results: The offspring of the women who were randomly assigned to a high volume of exercise in mid and late pregnancy were significantly lighter (3.39 kg vs 3.81 kg) and thinner (8.3% fat vs 12.1% fat) than those offspring born of women who were randomly assigned to reduce their exercise volume after the 20th week. Maternal weight gain, fresh placental volumes, and histomorphometric indices of placental function were greater in the high-low group. Conclusion: These data indicate that a high volume of moderate-intensity, weight-bearing exercise in mid and late pregnancy symmetrically reduces fetoplacental growth, whereas a reduction in exercise volume enhances fetoplacental growth with a proportionally greater increase in fat mass than in lean body mass. (Am J Obstet Gynecol 2002;186:142-7.)