Nuria, I apologize for referring to you in my previous response as Ellen - - it's 1:40am here, and although I'm a night person, clearly some synapses didn't connect properly!  

You asked about stress hormones:  "Can someone guide me to studies done on the effects of stress hormones in breast milk on infant’s health/wellbeing?"

This 2015 open access study from Duke University Medical Center in the U.S. is another one that I found interesting.  A quick PubMed search yielded 146 results using the search term, "cortisol in human milk".  https://www.ncbi.nlm.nih.gov/pubmed/26196472

Study title:  Correlation Between Maternal and Infant Cortisol Varies by Breastfeeding Status

Journal:  Infant Behavior & Development

Study authors: Benjamin Neelon SE, Stroo M, Mayhew M, Maselko J, Hoyo C.

Citation:  Infant Behav Dev. 2015 Aug;40:252-8. doi: 10.1016/j.infbeh.2015.06.005. Epub 2015 Jul 18.

Abstract:  
"OBJECTIVE: The objective of this study was to examine associations of mother and infant salivary cortisol, measured three times over the course of a day, and assess whether these varied by breastfeeding status."

"METHODS: We conducted a cross-sectional study of 54 mothers and their infants aged 4-11 months. Mothers collected their own saliva and that of their infants upon awakening, 30min after waking and at bedtime. Breastfeeding status was reported by mothers and cortisol level was measured in saliva in μg/dl using standard techniques. We used generalized linear models to evaluate relationships between maternal and infant cortisol levels, and assessed whether the relationship differed by breastfeeding status: formula only compared to partial and full breastfeeding, adjusting for infant sex, race, age, maternal education, and family income."

RESULTS: "Thirty-four infants received formula only and 20 were either partially or fully breastfed. Breastfeeding was associated with higher household income, higher maternal education, and white race. Cortisol levels were higher among breastfed infants at all three time points. After adjustment, maternal cortisol levels were related with infant cortisol at bedtime only (regression estimate 0.06; 95% CI: 0.10, 1.1; p=0.02). The adjusted association between bedtime maternal and infant cortisol was stronger among breastfeeding dyads than among formula-feeding dyads (regression estimate 1.0; 95% CI: 0.1, 2.0; p=0.04 vs. 0.6; CI: -0.1, 1.3; p=0.10). In addition, we assessed the influence of maternal education and household income in our adjusted model; income strengthened the observed association, whereas maternal education did not change the estimate."

CONCLUSIONS: "Breastfeeding mothers and infants had significant correlations for cortisol at bedtime, while formula-feeding dyads did not. These data suggest that several factors may contribute to cortisol synchrony observed in mother/infant dyads, including the transfer of cortisol in human milk, physical interaction such as skin-to-skin contact, and shared environment. In addition, our findings support household income as a possible contributor."

KEYWORDS:  "Breastfeeding; Cortisol; Human milk; Stress"

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