An assessment of hair cortisol among postpartum Brazilian mothers and infants from a high-risk community in São Paulo: Intra-individual stability and association in mother-infant dyads.

This study examined maternal-infant synchrony of hair cortisol at 12 months after birth and the intra-individual stability of maternal hair cortisol in the postpartum period. Participants were selected from an ongoing São Paulo birth cohort project, where families are considered to be "high-risk" due to their chronic stress experiences, with the majority living in slums (favelas). Cortisol was collected through 3-cm segments of hair samples, with values representing approximate levels of cortisol from 9 to 12 months for mothers and children and 6 to 12 months for mothers. Maternal and infant cortisol values reflecting chronic stress 9-12 months after birth were highly correlated (r = .61, p < .001); earlier maternal cortisol levels (6-9 months) and child cortisol levels at 9-12 months (r = .51, p < .001) were also correlated. Maternal cortisol values showed stability over time (r = .79, p < .001). These maternal-infant correlations are high compared to the existing literature on hair cortisol in other mother-child dyads, suggesting stronger synchrony under high-risk contexts where families are faced with challenging circumstances.