OBJECTIVES: Maternal nutritional status is closely related to fetal intrauterine development and an abnormal birth weight increases various disease risks across life stages. To better guide pregnancy weight gain, we aimed to explore the optimal weight gain for pregnant women with different body mass indexes (BMIs). METHODS: This retrospective cohort study included 68,981 women with singleton live birth between January 2017 and October 2021 in maternity centres in Shanghai, China. The fluctuations of the incidence of small and large for gestational age (small for gestational age (SGA) and LGA, respectively) were recorded at different maternal pre-pregnancy BMI (p-BMI) and different gestational weight gain (GWG) groups to find the lowest point of abnormal fetal weight incidence. The optimal GWG was then determined using a linear regression equation. RESULT: The lowest risk of LGA/SGA was associated with a maternal p-BMI of 19.46 kg/m CONCLUSION: By merely using the pre-pregnancy BMI, this study has established the optimal GWG equation, with the goal of achieving the appropriate fetal gestational age. It is a practical measure to ensure desirable pregnancy outcomes and meet the health economics requirements.