Population-based longitudinal evidence on body mass index (BMI) changes across successive pregnancies remains limited, despite its potential implications for long-term maternal and metabolic health1,2. Understanding patterns of interconception BMI change may help identify opportunities for preventive intervention. This study describes patterns of interconception BMI change and examines associations with selected sociodemographic factors among Tasmanian mothers from 2010 to 2022. This longitudinal population-based cohort study analysed Tasmanian perinatal records from public and private births between 2010 to 2022. Interconception BMI change between consecutive pregnancies was categorized as loss (>1 kg/m²), stable (between -1 and +1 kg/m²), moderate gain (>1 to 3 kg/m²), or high gain (≥3 kg/m²). Patterns of BMI change were summarised using descriptive statistics and graphical methods. Associations between BMI change and sociodemographic factors were examined using multinomial generalized estimating equations, accounting for repeated births within mothers. The cohort included 19,400 mothers with 45,922 deliveries. Mean BMI at first conception was 26.8 kg/m² and increased with parity, with the largest increase occurring from the first to second pregnancies (1.2 kg/m²). By the second pregnancy, 45.5% of women initially classified as healthy weight mothers had gained BMI. The overweight category was highly transitional, with 33.5% progressing to obesity by their second pregnancy and 46.8% by the fourth pregnancy. In multinomial models, mothers with obesity at parity two had almost threefold higher odds of high BMI gain than mothers with overweight (OR 2.96, 95% CI 2.31-3.79). Higher maternal age and socioeconomic disadvantage were also associated with a greater likelihood of BMI change across pregnancies. The first interconception period represents a critical period for maternal BMI change. Targeted support during this period, particularly for women experiencing socioeconomic disadvantage and at risk of adverse weight trajectories, may help reduce excess weight gain and inform more tailored approaches to postpartum care and health monitoring, with potential benefits for long-term maternal metabolic health.