The primary goals of the application are to investigate the effects of a motivationally-tailored, individually targeted 12-wk physical activity intervention on 1) risk of GDM in women at high risk of the condition, 2) serum biomarkers associated with insulin resistance, 3) and the adoption and maintenance of exercise during pregnancy. Secondary goals are to investigate the impact of the intervention on gestational weight gain and selected birth outcomes. The overall goal of the intervention is to encourage pregnant women to achieve the American College of Obstetricians and Gynecologists (ACOG) Guidelines for physical activity during pregnancy (30 minutes or more of moderate- intensity activity on most days of the week) through increasing walking and developing a more active lifestyle. The intervention draws from the theory of Stages of Motivational Readiness for Change and Social Cognitive Theory constructs for physical activity behavior and will take into account the specific social, cultural, economic, and physical environmental challenges faced by women of diverse socioeconomic and ethnic backgrounds. The application is innovative in being the first, to our knowledge, to test a physical activity intervention designed to prevent GDM among high risk women. The intervention protocol can readily be translated into clinical practice in underserved and minority populations.