Background: Most studies on the association of maternal pregnancy weight with offspring weight trajectory have a short follow-up time. This study aimed to explore the associations of maternal prepregnancy body mass index (BMI) and gestational weight gain (GWG) with childhood weight trajectories in a 7-year birth cohort. Methods: A total of 946 mother–child pairs (467 boys and 479 girls) from a longitudinal birth cohort in Tianjin City, China, were included in this study, ranging from pregnancy to offspring at 7 years. The outcome variable was defined as overweight or not overweight in offspring at the last round. A group-based trajectory model was applied to identify childhood BMI trajectory groups. Results: Five discrete BMI trajectory groups were identified and characterized as constant underweight (25.2%), constant normal weight (42.8%), and high or increasing trajectory [at risk of overweight (16.9%), progressive overweight (11.0%) and progressive obesity (4.1%)]. Maternal prepregnancy overweight was associated with 1.72 (95% CI 1.14–2.60, P = 0.01) to 4.02 (95% CI 1.94–8.36, P < 0.001) times the risk of all high or increasing trajectory groups, and excessive GWG was related to groups at risk of overweight [relative risk ratio (RRR) 2.09, 95% CI 1.27–3.46, P = 0.004] and progressive obesity (RRR 3.33, 95% CI 1.13–9.79, P = 0.029). Children in all high or increasing trajectory groups were associated with greater overweight risk at the last round [risk ratios (RRs) ranged from 3.54 (95% CI 2.53–4.95, P < 0.001) to 6.18 (95% CI 4.05–9.42, P < 0.001)]. Conclusion: Maternal prepregnancy overweight and excessive gestational weight gain were associated with increasing or high-level childhood body mass index trajectories as well as a greater risk of overweight at 7 years.