Moment-to-moment Bidirectional Associations Between Human Milk Glucocorticoids and Infant Behavior

Human milk composition varies between and within mothers and is impacted by both maternal and infant factors. Glucocorticoids in milk may potentially influence infant development. For example, elevated cortisol concentrations in milk have been associated with more infant negative affect. However, evidence supporting the role of human milk glucocorticoids on infant behavior is inconsistent. The current preregistered study investigated bidirectional associations between diurnal milk glucocorticoids and infant crying and sleep. We hypothesized that 1) higher cortisol and cortisone concentrations would be related to more infant crying and less infant sleep, and 2) more infant crying and less infant sleep would be related to higher cortisol and cortisone concentrations. At 6 weeks postpartum, healthy mothers (N=109) collected three milk samples: in the morning, afternoon, and evening. During this same day, mothers kept a logbook on infant crying and sleep. We calculated the duration of crying and sleeping over three time intervals: 1) the complete interval between each milk sample, 2) the 1.5 hours after each breast milk sample, and 3) the 1.5 hours before each milk sample. Next, we performed multilevel models to assess the bidirectional associations between cortisol and cortisone milk concentrations and infant crying and sleep. Against our hypotheses, we found that higher cortisol and cortisone milk concentrations were related to more infant sleep 1.5 hours after a milk sample. Moreover, more infant sleep 1.5 hours before a milk sample was related to higher cortisol and cortisone milk concentrations. No associations were found between milk glucocorticoids and infant crying. This study is the first to assess moment-to-moment associations between milk glucocorticoids and infant behavior and broadens our understanding of the role of human milk composition on infant behavior and vice versa. • We assessed bidirectional links between milk glucocorticoids and infant behavior. • Higher milk glucocorticoids were associated with more infant sleep after feeding. • More infant sleep before feeding was related to higher milk glucocorticoids. • Milk glucocorticoids were not associated with infant crying, nor vice versa.