Abstract: Objective To determine the associations of pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) with gestational diabetes mellitus (GDM). Methods A prospective cohort of pregnant women were screened for GDM at 24-28 weeks of gestation between 2013 and 2015, resulting in a sample of 3 593 with GDM and 15 346 without GDM. The body mass, plasma glucose, and height data of the participants were collected by the local medical workers. Multivariate logistic regression analyses were performed to determine the associations of pre pregnancy body mass index and weight gain during pregnancy with GDM. Results The participants with pre pregnancy overweight 〔odds ratio(OR)=2.44，95% cofidence interval(CI)1.98-2.99〕 and obesity (OR=4.98，95%CI 2.52-9.91) were more likely to develop GDM. According to the Institute of Medicine (IOM) criteria, excessive GWG in the first trimester occurred in 8.46% of the women, compared with 55.07% in the second trimester. After adjustment for age at delivery and pre pregnancy BMI, high GWG in the first trimesters in advanced maternal age (age at delivery≥35 yr.) group (OR=1.42，95%CI 1.02-2.28) was a risk factor for GDM while the OR value of the non-advanced maternal age (age at delivery≤35 yr.) group was not statistically significant. In second trimesters, both advanced maternal age group (OR=1.59，95%CI 1.14-1.88) and non-advanced maternal age group (OR=1.49，95%CI 1.20-1.72) in high GWG were associated with high risk of GDM. Conclusion Pre pregnancy overweight and obesity and excessive GWG during early and second trimesters of pregnancy may increase the risk of GDM in women in Southwestern China.