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Gestational weight gain (GWG) is a modifiable factor associated with maternal and neonatal outcomes. However, whether recommended weight gain ranges are appropriate across different pre-pregnancy body mass index (BMI) categories remains uncertain, particularly in Asian populations. This study examined the associations between GWG and maternal and neonatal outcomes across pre-pregnancy BMI categories using gestational weight management recommendations developed for the Chinese population. A retrospective cohort study was conducted in a public general hospital in Beijing, China. Pregnant women with singleton pregnancies who attended routine antenatal care and delivered between January 2021, and December 2024 were included. Women with incomplete data were excluded. GWG was calculated as the difference between self-reported pre-pregnancy weight and weight recorded before delivery. Weight gain was classified as inadequate, adequate, or excessive within each pre-pregnancy BMI category according to Chinese gestational weight management guidelines. Maternal outcomes included Caesarean section, non-elective caesarean, mode of birth, gestational diabetes, and gestational hypertension. Neonatal outcomes included preterm birth, low birthweight, and macrosomia, neonatal asphyxia. Multivariable logistic regression analyses stratified by pre-pregnancy BMI category were performed. A total of 3,734 women were included, of whom 66.1% had normal pre-pregnancy BMI. Among women with normal BMI, excessive GWG was associated with higher odds of caesarean section (adjusted odds ratio 1.38, 95% confidence interval 1.15–1.66) and non-elective caesarean Sect. (1.56, 1.16–2.11). Inadequate weight gain in this group was associated with increased odds of preterm birth (2.60, 1.46–4.49) and low birthweight (2.48, 1.37–4.32). Excessive GWG was also associated with higher odds of macrosomia among women with normal BMI (3.08, 1.98–4.92) and those who were overweight (2.96, 1.31–7.96). Among women with obesity, inadequate weight gain was associated with increased odds of preterm birth (6.28, 1.70–26.30). Other associations were limited or outcome-specific. Associations between GWG and maternal and neonatal outcomes varied across pre-pregnancy BMI categories. Women with normal pre-pregnancy BMI demonstrated the most consistent associations with adverse outcomes when gestational weight gain fell outside recommended ranges. These findings suggest that gestational weight management should not focus solely on women at the extremes of BMI, and support BMI-stratified approaches to antenatal weight monitoring and counselling using population-specific guidelines.