Objective: This study aims to identify lifestyle predictors of gestational diabetes mellitus (GDM) in South Asian women. Pre-gravid body mass index, body composition, rate of weight gain during pregnancy, physical activity (PA) and dietary intake were studied. Design: Prospective cohort study (n = 611) of South Asian women at Aga Khan University Hospital antenatal clinic in Karachi, Pakistan. Weight, height and body fat % were measured and data on PA, diet and socio-economic covariates were collected at baseline. A glucose screen was performed at 29.8±2.6 weeks. Results: Using logistic regression analysis for the full cohort, the risk of GDM increased with older maternal age (odds ratio (OR) 1.13, confidence interval (CI) 1.06-1.21), increased body fat % (OR 1.07, CI 1.03-1.13) and the risk was inversely associated with PA (100 kcal) (OR 0.79 0.99, CI 0.89). Using a nested case (n = 49) control (n = 98) design to study dietary intake, the risk of GDM decreased with increasing protein (% energy) in the diet (OR 0.75, CI 0.60-0.95). Conclusions: Body fat %, physical inactivity and, possibly, diet quality are important modifiable risk factors for GDM.