Objectives: Glucocorticoids are one component of human milk (HM) potentially affecting offspring development. Previous studies have identified various maternal, obstetric and socioeconomic characteristics that are associated with HM cortisol concentration but the literature is still scarce concerning these determinants in human populations. We aimed to identify which factors are linked with HM cortisol concentration at 2 months postpartum.

Methods: We analyzed data from 340 lactating Finnish mothers using ordinary least squares regression with log-transformed HM cortisol concentration as the dependent variable. Potential predictors included obstetric and maternal factors (maternal age, parity status, delivery mode, gestational age, pre-pregnancy obesity, and smoking in pregnancy), socioeconomic status (education and socioeconomic class), subjective economic well-being, maternal psychosocial factors (postpartum depression and anxiety symptoms), infant sex and age, and HM sample characteristics (time of the day and season of the year at sample collection).

Results: The strongest and most robust predictors were season of the year of sample collection and parity status. HM cortisol concentration was significantly higher for primiparas than multiparas. HM samples collected in summer showed significantly higher cortisol concentrations than those collected in winter, spring or autumn.

Conclusions: The findings suggest that parity and season of the year at sample collection may be important factors to control for when examining HM cortisol. The strongest and most robust associations were related to maternal and sample characteristics and not to socioeconomic and psychosocial distress. This may be related to the fact that the study was conducted in a low-risk population.