Maternal adiposity confers an increased risk of GDM in pregnancy. A low glycemic index (GI) dietary intervention has been found to improve glucose homeostasis and reduce gestational weight gain. Mobile Health (mHealth) Technology-assisted interventions are becoming commonplace as an aid to treating many chronic diseases. The aim of this study is to assess the impact of a 'healthy lifestyle package' with mHealth smart phone technology as support compared with usual care on the incidence of GDM in an overweight and obese pregnant population. We propose a randomized controlled trial of an mHealth assisted healthy lifestyle intervention package versus standard obstetric care in pregnant women with a BMI ≥25kg/m(2)-39.9kg/m(2). Patients are randomized to control or intervention group in a 1:1 ratio. The intervention arm healthy lifestyle package includes a motivational counseling session to encourage behavior change, involving targeted, low GI nutritional advice and daily physical activity prescription delivered before 18weeks gestation, as well as a smart phone app to provide ongoing healthy lifestyle advice and support throughout pregnancy. The primary outcome is the incidence of GDM at 29weeks' gestation and power analysis indicates that 253 women are required in each group to detect a difference. This will be the first clinical trial to evaluate the effectiveness of a smart phone technology-assisted targeted healthy lifestyle intervention, which is grounded in behavior change theories and techniques, to support antenatal management of an overweight and obese pregnant population in preventing GDM.