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Background Cardiovascular disease (CVD) is the leading cause of mortality worldwide. While established evidence links breastfeeding practices to reduced cardiometabolic risk in children, key gaps remain in understanding the long-term effects of breastfeeding. Specifically, it is unclear when these protective associations first emerge, how they evolve over time, and whether the benefits observed in early childhood persist into adolescence and early adulthood. This study aims to examine the association between breastfeeding duration and cardiometabolic risk factor trajectories from childhood to early adulthood.Methods Data were from the Avon Longitudinal Study of Parents and Children (ALSPAC), a prospective birth cohort study in southwest England, born in 1991/1992. Breastfeeding duration was measured using self-completed questionnaires at 6 months and 15 months post-partum. Cardiometabolic risk factors were measured from birth/mid-childhood to age 25 years (y) and included fat and lean mass (9y–25y), systolic and diastolic blood pressure (SBP, DBP), pulse rate and glucose (7y–25y), high-density lipoprotein cholesterol (HDL-c), non-HDL-c and triglycerides (birth-25y). We examined the sex-specific associations between breastfeeding duration (Never, <1 month, <3 months, <6 months, <9 months, 9+ months) and cardiometabolic risk factor trajectories using linear spline multilevel models, stratified by sex. The models were adjusted for sociodemographic confounders, including self-reported maternal baseline characteristics (ethnicity, marital status, social class, education, pre-pregnancy BMI, gravidity, and smoking during pregnancy).Results Among 2,507–5,057 participants with 7,833–32,061 repeated measures, associations between breastfeeding duration and fat mass were evident at age 9y in females, showing a graded relationship, while in males, associations emerged from 18y onwards. Associations between breastfeeding duration and lean mass in both sexes strengthened at age 13y and persisted until 25y. Graded associations between breastfeeding duration and SBP and DBP trajectories became apparent between ages 16y–18y and widened further by age 25y, particularly among males. Associations with lipid trajectories emerged in both sexes, with differences in HDL-c observed between ages 18y–25y, non-HDL-c differences appearing between ages 9y–18y, and triglyceride differences evident between ages 9y–18y, particularly among females. Associations with glucose levels emerged among males only, between ages 15y–25y.Conclusion These findings suggest that the cardiometabolic benefits of breastfeeding emerge in late childhood and adolescence and persist into early adulthood. Early life targeted interventions promoting sustained breastfeeding practices may have the potential to reduce cardiovascular risk in early adulthood.