Stress hormones in breast milk do not appear to increase infant crying. Instead, they may be linked to more sleep shortly after feeding. That is the main finding of a new study by researchers from the Developmental Psychobiology Lab at Radboudumc’s Department of Medical Neuroscience.
Human milk contains cortisol and cortisone, hormones often associated with stress. Their levels fluctuate throughout the day and can rise when mothers experience stress. This has raised concerns among parents about whether stress could be transmitted to infants through breast milk and influence their behavior.
Tracking milk hormones and infant behavior
To examine this question, researchers studied 109 healthy mothers. Participants collected milk samples in the morning, afternoon, and evening of a single day. The team measured cortisol and cortisone levels in each sample. Mothers also recorded their infants’ sleep and crying patterns throughout the day.
The researchers analyzed whether hormone levels in milk were associated with infant behavior after feeding. They also examined whether infant sleep or crying before milk sampling was linked to hormone levels in milk.
More sleep, no increase in crying
Higher cortisol and cortisone levels in milk were associated with more infant sleep in the 1.5 hours following feeding. Infants who had slept more in the 1.5 hours before milk sampling also had mothers with higher milk hormone levels.
PhD candidate and first author Nina Bruinhof: ‘We found no association between stress hormones in breast milk and infant crying. We also confirmed a clear daily rhythm in cortisol and cortisone levels, with higher concentrations in the morning and lower levels in the evening.’
What do these findings mean?
The results suggest that, in healthy mothers, naturally occurring stress hormones in milk do not have apparent negative effects on infant crying or sleep. The nature of the relationship remains unclear. ‘It is possible that milk hormones influence infant sleep, that infant sleep affects hormone levels, or that both are shaped by daily routines and other factors,’ says Bruinhof.
Further research is needed to clarify the underlying mechanisms. For now, the findings provide reassurance that normal fluctuations in stress hormones in breast milk are not associated with adverse behavioral effects in infants.
About the publication
Bruinhof, N., Beijers, R., & de Weerth, C. (2026). Moment-to-moment bidirectional associations between human milk glucocorticoids and infant behavior. Psychoneuroendocrinology, 187, 107790. https://doi.org/10.1016/j.psyneuen.2026.1077the s90
