OBJECTIVE To study the impacts of pre-pregnancy maternal BMI and gestational weight gain(GWG) on pregnancy outcomes. METHODS We adopted a prospective cohort study with cluster sampling in single pregnant women, who were not with hypertension, diabetes, hyperlipidemia or other diseases in the previous history, neither did they have diseases of heart, liver, kidney, thyroid etc. related to current pregnancy. Those pregnant women who visited the prenatal nutrition clinic under 'informed consent' were surveyed with questionnaire to track their peri-natal complications, delivery mode and neonate birth outcomes etc. Pearson and partial correlations, chi-square test and binary logistic regression were used to study the association between pre-pregnancy maternal BMI, GWG and pregnancy outcomes. RESULTS A total of 623 pregnant women were recruited in the cohort, with 592 (95%) of them eligible for analysis. Results from the Multivariate Logistic Regression analysis indicated that, after controlling the potential confounding factors, when compared to women with pre-pregnancy BMI between 18.5 and 24.0, the odds ratios (ORs) for low birth ponderal index (PI) were 2.34 [95% confidence interval (CI), 1.24-4.42)]among those with BMI<18.5, respectively, while 2.73 (1.12-6.68) for high birth PI among those with BMI > 24.0. Similarly, when compared to pregnant women with normal GWG(defined as weight gain range from P15 to P85 by stratification of pre-pregnancy BMI), low GWG (<P15) seemed to be the risk factor for preterm birth, low birth weight, gestational diabetes mellitus, with low birth PI [ORs as 4.85(1.35-17.51), 10.30 (2.29-46.35), 2.29 (1.07-4.93) and 2.65(1.24-5.68), respectively. High GWG (>P 85)appeared the risk factor for high birth weight, high birth PI, and gestational diabetes mellitus, with ORs as 3.83(1.74-8.44), 2.39(1.14-5.01)and 2.21(1.07-4.55), respectively. CONCLUSION Low or high pre-pregnancy maternal BMI and GWG were associated with adverse pregnancy outcomes.