Background: Overweight and obesity is a significant health concern during pregnancy. Our aim was to investigate the effect of providing antenatal dietary and lifestyle advice to women who are overweight or obese on components of maternal diet and physical activity.Methods: We conducted a randomised controlled trial, in which pregnant women with a body mass index ≥25 kg/m 2 , and singleton gestation between 10 +0 to 20 +0 weeks were recruited and randomised to Lifestyle Advice (involving a comprehensive dietary and lifestyle intervention over their pregnancy) or Standard Care. Within the intervention group, we conducted a nested randomised trial in which a subgroup of women were further randomised to receive access to supervised group walking sessions in addition to the standard information presented during the intervention contacts (the Walking group) or standard information only.The outcome measures were maternal dietary intake, (including food groups, macronutrient and micronutrient intake, diet quality (using the Healthy Eating Index; HEI), dietary glycaemic load, and glycaemic index) and maternal physical activity. Women completed the Harvard Semi-Structured Food Frequency Questionnaire, and the Short Questionnaire to Assess Health-enhancing Physical Activity (SQUASH), at trial entry, 28 and 36 weeks' gestational age, and 4 months postpartum.Analyses were performed on an intention-to-treat basis, using linear mixed effects models with adjustment for the stratification variables.Results: Women randomised to Lifestyle Advice demonstrated a statistically significant increase in the number of servings of fruit and vegetables consumed per day, as well as increased consumption of fibre, and reduced percentage energy intake from saturated fats (P < 0.05 for all). Maternal HEI was significantly improved at both 28 (73.35 ± 6.62 versus 71.86 ± 7.01; adjusted difference in means 1.58; 95% CI 0.89 to 2.27; P < 0.0001) and 36 (72.95 ± 6.82 versus 71.17 ± 7.69; adjusted difference in means 1.77; 95% CI 1.01 to 2.53; P < 0.0001) weeks. There were no differences in dietary glycaemic index or glycaemic load. Women randomised to Lifestyle Advice also demonstrated greater total physical activity (adjusted difference in means 359.76 metabolic equivalent task units (MET) minutes/week; 95% CI 74.87 to 644.65; P = 0.01) compared with women receiving Standard Care. The supervised walking group was poorly utilised.Conclusions: For women who are overweight or obese, antenatal lifestyle advice improves maternal diet and physical activity during pregnancy.