The aim of this study was to investigate associations between pre-pregnancy body mass index (BMI) and adverse pregnancy outcomes among Chinese pregnant women. A prospective population-based cohort study was performed using data collected as part of the China-Anhui Birth Cohort Study or C-ABCS. A total of 13,121 pregnant women who received the first prenatal visit were enrolled from November 2008 to October 2010. Logistic regression analysis was used to calculate associations between pre-pregnancy BMI and pregnancy outcomes. Results indicated that the increased pre-pregnancy BMI was associated with a number of adverse pregnancy outcomes such as hypertensive disorder (adjusted relative risk (ARR) 2.3, 95% confidence interval [CI] 1.5-3.6), gestational diabetes (ARR 3.5, 95% CI 2.3-5.2), caesarean delivery (ARR 2.0, 95% CI 1.6-2.4), and medically indicated preterm delivery (ARR 1.8, 95% CI 1.1-2.9). Women with pre-pregnancy BMI above the normal range pose an increased risk of adverse pregnancy outcomes.