Intra-individual stability and developmental change in hair cortisol among postpartum mothers and infants: Implications for understanding chronic stress.

The study goal was to determine the intra-individual stability, developmental change, and maternal-reported correlates (socio-demographic, stress experiences, hair characteristics, and care) of hair cortisol in mothers and their infants. To assess cortisol deposition in hair during the periods of 6-to-9 months and 9-to-12 months of age, 3 cm segments of hair samples deemed to represent approximately 3 months of retrospective hair cortisol were sampled longitudinally at 9- and 12-months in 41 mothers and infants. Bivariate correlations and mean level comparisons of log-transformed hair cortisol levels at 9- (T1) and 12-months (T2) in mothers and infants were examined. Hair cortisol values were positively correlated from T1 to T2 for mothers (r = .41, p < .05) and infants (r = .39, p < .05). Hair cortisol values did not significantly differ from T1 to T2 in infants but decreased for mothers (F(1,34) = 9.2, p < .01). Maternal and infant hair cortisol was not associated with each other at either time point. Self-reported measures of stress, and hair characteristics and care were not associated with hair cortisol. This is the first study to obtain hair cortisol from more than one time point within the first year after birth in mothers and infants. The intra-individual stability of hair cortisol suggests that it may be a possible biomarker for detecting change in chronic stress experiences within the first year of life and in the postpartum period.