The prevalence of overweight and obesity amongst reproductive women has been increasing worldwide. Our aim was to compare pregnancy outcomes and infant neurocognitive development by different BMI classifications and investigate whether early pregnancy BMI was associated with risks of adverse outcomes in a Southwest Chinese population. We analysed data from 1273 women enrolled in the Complex Lipids in Mothers and Babies (CLIMB) randomized controlled trial in Chongqing, China. Maternal BMI was classified as underweight, normal weight and overweight/obese according to the Chinese, WHO Asian, and WHO European standards. For the adverse pregnancy outcomes, after adjustment for potential confounders, an underweight BMI was associated with increased risk of small for gestational age (SGA) babies, and an overweight/obese BMI was associated with increased risk of maternal gestational diabetes mellitus (GDM), caesarean section (C-section), macrosomia and large for gestational age (LGA) babies. For infant neurocognitive development, 1017 mothers and their children participated; no significant differences were seen in the Mental Development Index (MDI) or the Psychomotor Development Index (PDI) between the three BMI groups. Our findings demonstrate that abnormal early pregnancy BMI were associated with increased risks of adverse pregnancy outcomes in Chinese women, while early pregnancy BMI had no significant influence on the infant neurocognitive development at 12 months of age.