In a cross-sectional study, 163 breastfeeding women completed the Edinburgh Postnatal Depression Scale (EPDS), a questionnaire on demographics and infant feeding and hand-expressed breast milk for Na and K quantification, between 2 and 12 weeks postpartum. Forty women (24.5%) had an EPDS score compatible with the risk of a depressive episode, and 63 (41%) did not feel confident about breastfeeding. These 2 variables were significantly correlated to each other and individually correlated to breastfeeding exclusiveness. Weeks postpartum was correlated to breastfeeding exclusiveness and Na:K in milk (all P < .001). A logistic regression model showed that supplementation increased the risk of high Na:K in milk by 209%, whereas a longer time postpartum lowered the risk for mammary gland permeability. This study suggests that postpartum depression and low breastfeeding confidence, which may be present concomitantly, are associated with increased mammary gland permeability, only to the extent in which depression dissuades the mother from exclusive breastfeeding.