Objective: To evaluate whether an intensive, medically supervised exercise intervention improved maternal glycemia and gestational weight gain in obese pregnant women when compared with routine prenatal care., Methods: This randomized controlled trial compared a medically supervised exercise intervention with routine prenatal care. The primary outcome was a reduction in mean maternal fasting plasma glucose in the intervention group by 6.9 mg/dL at the time of a 75-g oral glucose tolerance test at 24-28 weeks of gestation. Secondary outcomes included excessive gestational weight gain. The intervention consisted of 50-60 minutes of exercise: warm-up, resistance or weights, aerobic exercises, and cool-down. All women received routine prenatal care. Power calculation determined that 24 women were required per group to detect a difference of 6.9 mg/dL in fasting plasma glucose between groups based on an independent-sample t test for statistical power of 80% at a type I error rate of 0.05. A sample size of 44 per group was planned to allow a dropout rate of 33%., Results: From November 2013 through August 2015, 88 women were randomized: 44 each to the exercise and control groups. Eight women in the control group and 11 in the intervention group did not complete the trial at 6 weeks postpartum (P=.61), but 43 in each group attended the 24- to 28-week glucose screen. There were no baseline maternal differences between groups. Classes commenced at a mean of 13 4/7±1 2/7 weeks of gestation. In early pregnancy, 51.1% (n=45/88) had an elevated fasting plasma glucose (92-125 mg/dL). There was no difference in the mean fasting plasma glucose at 24-28 weeks of gestation: 90.0±9.0 mg/dL (n=43) compared with 93.6±7.2 mg/dL (n=43) (P=.13) or in the incidence of gestational diabetes mellitus at 24-28 weeks of gestation: 48.8% (n=21/43) compared with 58.1% (n=25/43) (P=.51) in the control and exercise groups, respectively. At 36 weeks of gestation, excessive gestational weight gain greater than 9.1 kg was lower in the exercise group, 23.5% compared with 45.2% in the control group (P<.05)., Conclusion: An intensive, medically supervised exercise intervention for obese women from early pregnancy did not improve maternal glycemia. Pregnant women who are obese, however, should be advised to exercise because it attenuates excessive gestational weight gain., Clinical Trial Registration: International Standard Randomised Controlled Trials (ISRCTN) registry, ISRCTN 31045925.