Context: Epidemiological studies have identified the importance of the in utero environment in providing a healthy start to life. Previous studies have suggested that the maternal environment, in particular a reduction in maternal insulin sensitivity, contributes significantly to fetal growth. Regular aerobic exercise, through an effect on maternal insulin sensitivity, may influence offspring size by regulating nutrient supply to the fetus. Objective: The aim of the study was to determine the effects of aerobic exercise training in the second half of pregnancy on maternal insulin sensitivity and neonatal outcomes. Design and Setting: We conducted a community-based, randomized, controlled trial of exercise in pregnancy. Participants: Eighty-four healthy nulliparous women (mean ± sd, age, 30 ± 4 yr; body mass index, 25.5 ± 4 kg/m2) participated in the study. Intervention: Subjects participated in a home-based stationary cycling program from 20 wk gestation to delivery. Main Outcome Measures: Maternal insulin sensitivity, neonatal auxology, body composition, and growth-related peptides in cord blood were measured. Results: Offspring of exercisers had lower birth weight (sd score, control, 0.23 ± 0.8; exercise, −0.19 ± 0.9; P = 0.03) and body mass index at birth (sd score, control, 0.40 ± 0.9; exercise, −0.01 ± 0.09; P = 0.04). The reduction in maternal insulin sensitivity in late gestation was not affected by exercise (P = 0.45) and was unrelated to offspring size. Exercise offspring had lower cord serum IGF-I (P = 0.03) and IGF-II (P = 0.04). Conclusions: Regular exercise was associated with lower birth weights and reduced cord concentrations of growth-related peptides, suggesting an influence of exercise on endocrine regulation of fetal growth. These effects on offspring growth were not associated with an exercise training effect on maternal insulin sensitivity. Regular exercise during pregnancy is associated with lower birth weight and reduced cord IGF concentrations, without exercise-associated alterations in maternal insulin sensitivity.