Decreased risk of fetal macrosomia in this group of women is likely to reflect the effect of GDM treatment. Aims: To evaluate the effect of maternal body mass index (BMI) on gestational diabetes (GDM) and the risk of adverse pregnancy outcomes in women who are overweight or obese. Methods: A prospective cohort study nested within the LIMIT randomised controlled trial. A total of 1030 women were recruited between 10 and 20 weeks' gestation, with a BMI≥25kg/m2, and were grouped into BMI subclasses utilising World Health Organisation criteria. Women underwent a fasting oral glucose tolerance test at 26-28 weeks' gestation, and a diagnosis of GDM was made if fasting blood glucose was ≥5.5mmol/L or ≥7.8mmol/L after 2h. Maternal and neonatal health outcomes were evaluated. Results: The prevalence of GDM increased with increasing maternal BMI (6.74% overweight vs 13.42% obese subclass 1 vs 12.79% obese subclass 2 vs 20.00% obese subclass 3). Women who were diagnosed with GDM were significantly less likely to give birth to an infant with birth weight above 4. kg (RR 0.60; 95% CI 0.36 to 1.00; p=. 0.05). The need for caesarean delivery (RR 1.27; 95% CI 1.07 to 1.50; p=. 0.006) and incidence of birth weight >90% (RR 1.38; 95% CI 1.07 to 1.77; p=. 0.01) was significantly increased in women who were obese, independent of GDM. Conclusion: Increasing maternal BMI is a significant risk factor for the development of GDM, and our findings demonstrate a considerably higher prevalence than has been previously described. Raised maternal BMI is a risk factor for high infant birth weight, which may be modified by lifestyle intervention.