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Previous studies provided contradictory results regarding the influence of maternal, seasonal, and infant factors on breastmilk cortisol, and its associations with breastmilk composition and infant development. This study aimed to assess breastmilk cortisol levels at the first, third, and sixth months of lactation and evaluate the associations with maternal psychosocial, seasonal, and infant factors, breastmilk composition, and infant anthropometric and psychomotor development and temperament. Cortisol concentrations were assessed by ELISA in 24 h breastmilk samples obtained from 38 healthy mothers. Maternal psychological status was assessed by EPDS and PSS-10 and infant psychomotor development was assessed using the Children’s Development Scale (DSR). Breastmilk cortisol was 11.2 ± 6.2, 11.2 ± 4.3, and 12.7 ± 6.2 ng/mL at the first, third, and sixth months of lactation (p > 0.05), respectively. In the spring-summer season, we observed lower and higher levels of cortisol in the first and sixth months of lactation (p ≤ 0.05), respectively, but no other associations were detected regarding maternal or infant characteristics. In the third month of lactation, cortisol was related to breastmilk crude protein (β = 0.318, 0.007–0.630) and infant BMI z-score before adjustment for infant birthweight and sex (Model 2: β = 0.359, 0.021–0.697), but no other associations with breastmilk composition, infant development, or temperament were confirmed. Our results indicated that breastmilk cortisol is unrelated to maternal and infant factors and has limited influence on breastmilk crude protein, but not on infant anthropometric and psychomotor development.