The study was to assess the associations of prepregnancy body mass index (BMI) and gestational weight gain (GWG) with pregnancy outcomes. This was a retrospective analysis of 2973 nulliparous women who delivered single live babies. Prepregnancy BMI was categorized as underweight (<18.5 kg/m(2)), normal weight (18.5-24.9 kg/m(2)) or overweight/obese (>= 25.0 kg/m(2)). GWG was categorized as inadequate, adequate or excessive. 567 (19.1%) women were overweight/obese, and 1600 (53.8%) exhibited excessive GWG. Compared with women of normal weight, overweight/obese women had a higher incidence of cesarean section (odds ratio, 95% confidence interval: 2.02, 1.59-2.56), postpartum hemorrhage (1.50, 1.05-2.14), preterm delivery (2.51, 1.83-3.45), preterm premature rupture of membranes (2.11, 1.32-3.38), gestational diabetes mellitus (2.04, 1.65-2.53), gestational hypertension (7.68, 4.21-14.00), preeclampsia (1.98, 1.18-3.33) and small for gestational age (2.81, 1.21-6.54). Compared with adequate GWG, excessive GWG increased the incidence of cesarean section (2.02, 1.59-2.56), preterm delivery (1.48, 1.05-2.71), preeclampsia (1.78, 1.34-4.27) and macrosomia (2.61, 1.61-4.25), and reduced the incidence of gestational diabetes mellitus (0.75, 0.62-0.92). High prepregnancy BMI and excessive GWG in nulliparous Chinese women are associated with adverse pregnancy outcomes. Weight control before and during pregnancy could reduce the complications of pregnancy.