The goal of this trial is to investigate the efficacy of a lifestyle intervention of healthy diet and physical activity for helping overweight/obese pregnant women to achieve appropriate gestational weight gain (GWG) for their prepregnancy body mass index (BMI). The goal of the intervention is to help women achieve the lower bound of the GWG range recommended by the Institutes of Medicine (IOM) for a given prepregnancy BMI category (i.e., 11 lbs for obese women and 15 lbs for overweight women). The lifestyle intervention will be delivered via 2 in-person counseling sessions and 11 telephone contacts with study dieticians trained in motivational interviewing techniques. The lifestyle intervention will be compared to usual medical care. Outcomes will be assessed by trained study personnel at approximately 10 weeks and 32 weeks gestation, within several days of delivery and at 6 months and 12 months postpartum.