Ways to prevent gestational diabetes mellitus (GDM) remain unproven. We compared the impact of three lifestyle interventions (healthy eating HE, physical activity PA, and both HE and PA HE+PA) on GDM risk in a pilot multicenter randomized trial. Pregnant women at risk for GDM (BMI ≥29 kg/m2) from nine European countries were invited to undertake a 75-g oral glucose tolerance test before 20 weeks' gestation. Those without GDM were randomized to HE, PA, or HE+PA. Women received five face-to-face and four optional telephone coaching sessions, based on the principles of motivational interviewing. A gestational weight gain (GWG) <5 kg was targeted. Coaches received standardized training and an intervention toolkit. Primary outcome measures were GWG, fasting glucose, and insulin sensitivity (HOMA) at 35-37 weeks. Among the 150 trial participants, 32% developed GDM by 35-37 weeks and 20% achieved GWG <5 kg. HE women had less GWG (-2.6 kg 95% CI -4.9, -0.2; P = 0.03) and lower fasting glucose (-0.3 mmol/L -0.4, -0.1; P = 0.01) than those in the PA group at 24-28 weeks. HOMA was comparable. No significant differences between HE+PA and the other groups were observed. An antenatal HE intervention is associated with less GWG and lower fasting glucose compared with PA alone. These findings require a larger trial for confirmation but support the use of early HE interventions in obese pregnant women.