Hypothalamic-pituitary-adrenal activity in postpartum mothers : the role of infant feeding type, parity, salience of stressor and sociodemographic factors

Hypothalamic-pituitary-adrenal (HPA) responsiveness to stress is generally blunted in lactating rats, except during situations of pup endangerment, suggesting an important role of salience of stressor in stress response during lactation. Conversely, in women, breastfeeding is associated with reduced HPA responses to physical stress and perceived stress. Yet, HPA responses to a standardized speech and arithmetic task (Trier Social Stress Test, TSST) are not blunted in breastfeeding mothers. Interestingly, greater number of children (parity) increases maternal experience which can enhance responsiveness to child-related stressors. To our knowledge, no study to date has investigated the impact of infant feeding type and parity on basal and reactive HPA secretion in human mothers using an emotional stressor evoking threat to a child. Furthermore, no study has investigated the effect of return of ovarian activity, sleep disturbances, education and income even though these variables are function of infant feeding type. Therefore, in STUDY (1) we assessed the impact of infant feeding type (breast vs. bottle) and parity (primiparous, first-time mother vs. multiparous, second-time mother) on diurnal cortisol secretion. Then, in STUDY (2) we verified whether infant feeding upon waking up would affect awakening cortisol response (ACR), an endogenous stimulation of HPA axis. Finally, in STUDY (3) we investigated the impact of infant feeding type, parity and salience of stressor on salivary cortisol and (sympathetic) alpha-amylase responses to laboratory-induced (exogenous) stressors that were either related or unrelated to child well-being (a home-made emotional film evoking lost and hurt children or the TSST). Breastfeeding mothers presented lower estradiol concentrations, more frequent nocturnal sleep interruptions and higher education and income. We found that greater basal cortisol secretion in multiparous bottlefeeding compared to multiparous breastfeeding mothers at awakening and 4PM, when simultaneous child and infant cares are the most prominent. No impact of breastfeeding was found on ACR. Finally, among multiparous mothers, following both the TSST and the emotional film, cortisol responsiveness in bottlefeeding was greater than in breastfeeding mothers. Alpha-amylase responses were similar across groups. Interestingly, income played a role in the stability of diurnal cycle, and possibly of ACR across days, but not on laboratory-induced stress responses. No effect of other secondary variables was detected. In conclusion, in this dissertation, we show for the first time that during child-related stress, multiparity accentuates the reduction in basal and reactive HPA activity associated with breastfeeding. Furthermore, the evidence of potential association between high socioeconomic indices and healthier HPA function represent a new avenue for implementing support programs for mothers during the highly emotional and stressful period following childbirth