Background: Maternal psychological stress is common among mothers of hospitalized neonates and may be associated with impaired lactation. This study aimed to assess the association between maternal psychological stress and breast milk production among mothers of hospitalized neonates in the neonatal intensive and intermediate care units of Ramiro Prialé Prialé National Hospital, Peru, during 2021–2022 .

Methods: An observational analytical cross-sectional study was conducted among 239 mothers using the Parental Stressor Scale: Neonatal Intensive Care Unit (PSS). A score ≥ 64 indicated high stress. Milk production was measured on postpartum day seven and categorized as high (>180 ml/kg/day), medium (90–180 ml/kg/day), or low (<90 ml/kg/day). Group comparisons (high vs. low stress) used the Mann-Whitney U test. Spearman correlation and multivariate regression were applied to evaluate associations. Outliers were evaluated using boxplot inspection and standardized residuals.

Results: Mean milk production was 121.7 ± 23.3 ml/kg/day. Mothers with high stress had significantly lower milk production than low-stress mothers (U = 13.984, p < 0.001). Spearman's correlation showed a strong negative association (R = –0.964; 95% CI: –0.972 to –0.953; p < 0.001). Multivariate analysis adjusting for potential confounders (maternal age, gestational age, birth weight, delivery mode, parity) showed stress remained an independent associated of milk production (β = –0.82; p < 0.001).

Conclusion: Higher maternal psychological stress levels were significantly associated with lower breast milk production, but causality cannot be inferred. Findings highlight the need for improved psychological support in NICU settings.