Background <p>Exclusive breastfeeding during the first six months of life provides unmatched nutritional and immunological benefits, supporting optimal growth and development. However, maternal nutritional status may influence both the quantity of milk produced and infant growth trajectories. This study explores the relationship between maternal body mass index, breast milk intake, and infant body composition at three months of age. </p> Methods <p>81 healthy mother-infant dyads were recruited from multiple primary healthcare centers. Anthropometric data were collected for mothers and infants, and breast milk intake was assessed using the dose-to-mother deuterium oxide dilution method. Infant body composition was evaluated through stable isotope techniques.</p> Results <p>Exclusively breastfed infants exhibited significantly higher mean weight and length compared to mixed-fed peers, alongside greater milk intake volumes and fat mass percentages. Notably, infants born to mothers with a BMI ≥ 25 showed increased milk intake (mean 991 mL/day) and higher relative fat mass compared to infants of mothers with BMI &lt; 25. While this association attenuated after adjustment for confounders, a trend persisted, suggesting a potential link between maternal adiposity and lactation output or infant appetite regulation.</p> Conclusions <p>These findings highlight the dual relevance of promoting exclusive breastfeeding and supporting maternal health during the postpartum period. Maternal BMI appears to influence both infant nutritional intake and early adiposity, underscoring the need for targeted clinical strategies addressing overweight and obesity in lactating women. Early interventions focused on nutritional guidance and breastfeeding support may enhance lactation outcomes and contribute to healthy infant growth trajectories. Further longitudinal studies are warranted to elucidate the underlying biological mechanisms and long-term health implications.</p>