The primary findings of our study were that lower salivary cortisol levels after breastfeeding, higher breastfeeding self-efficacy, and absence of breast complications at 1 month postpartum were possible modifiable predictors of exclusive breastfeeding at 3 months postpartum.
Lower cortisol levels after breastfeeding were associated with subsequent exclusive breastfeeding practices. Notably, the relationship was observed despite no cross-sectional relationship observed between cortisol levels and exclusive breastfeeding at 1 month postpartum. Cortisol levels usually decrease after breastfeeding as oxytocin inhibits cortisol secretion in women without mood distress . Nevertheless, more than 40% of our participants had increased cortisol levels after breastfeeding than before. High cortisol levels after breastfeeding despite oxytocin action seem to reflect physiological and psychological stress related to breastfeeding. Previous studies reported no relationship between perceived stress and milk volume, although oxytocin levels were found to be decreased in women with psychological stress compared to the levels in those without psychological stress [12, 26]. Thus, we hypothesize that breastfeeding-related stress affects subsequent exclusive breastfeeding practice through psychological burden and associated behavioral changes, but not through fundamental physiological changes in lactation such as milk volume reduction. A stress response after breastfeeding is a modifiable factor. Approaches to stress reduction including prevention of breast complications, anxiety relief by providing advice about breastfeeding concerns, and relaxation therapy during breastfeeding could be effective . In addition, maternal cortisol responses to breastfeeding vary according to the function of the CD38 rs3796863 , an ectoenzyme that mediates the release of oxytocin. A recent study has indicated that the CD38 rs3796863 CC genotype is associated with reduced oxytocin release during breastfeeding and, accordingly, fewer cortisol-reducing responses to breastfeeding . The specific gene influences the association between breastfeeding and its cortisol responses through reduced oxytocin secretion, not through breastfeeding-related psychological stress. Thus, an association between post-breastfeeding cortisol levels and subsequent breastfeeding might be attenuated by not considering the effects of the gene. Further studies are needed to clarify the associations.
A higher BSES-SF score was associated with subsequent exclusive breastfeeding in the present study, as reported by a previous study of another population . Breastfeeding self-efficacy reflects a mother’s confidence in her ability to breastfeed her infant. The BSES-SF contains the following items “I can always keep wanting to breastfeed” and “I can always be satisfied with my breastfeeding experience” [10, 19]. Women with positive answers to such items seemed less stressed about breastfeeding. Women with lower breastfeeding self-efficacy are more likely to perceive milk insufficiency [10, 31]. Such a perception has been described as a factor related to cessation of exclusive breastfeeding . The perception of milk insufficiency itself was not assessed in our study because this concept was supposed to be contained in questions of the BSES-SF. A further detailed study regarding the relationship between breastfeeding self-efficacy, perception of milk insufficiency, and subsequent exclusive breastfeeding might contribute to the suggestion of concrete intervention methods for increasing the rate of exclusive breastfeeding. In the analyses according to parity, the association between breastfeeding self-efficacy and exclusive breastfeeding was observed only in multiparas. A previous study showed that the effect of self-efficacy on breastfeeding practice was much stronger in multiparas than primiparas ; this was because breastfeeding experiences in primiparas were affected more by subjective norms and social environment than by breastfeeding self-efficacy . Therefore, the effectiveness of improving breastfeeding self-efficacy on breastfeeding practices might differ according to parity. A systematic review showed that prenatal and postpartum interventions focusing on improving breastfeeding self-efficacy lead to exclusive breastfeeding . However, in Japan, the effects of prenatal intervention using a breastfeeding self-efficacy workbook are limited and effective only in baby-friendly hospitals and during the early postpartum period . The development of more effective approaches taking parity into consideration are needed for postpartum Japanese women, based on the breastfeeding self-efficacy theoretical framework by Dennis ; for instance, the elements such as performance accomplishments, vicarious experience, verbal persuasion, and physiological and emotional states in the theoretical framework would help in the development of self-efficacy expectations .
In multiparas, absence of breast complications at 1 month postpartum was associated with subsequent exclusive breastfeeding. Although this association supported the findings of previous studies [36, 37], we did not ascertain the reason why it was observed only in multiparas. In general, primiparas experience more breast complications than multiparas , as was also observed in the present study. Furthermore, breast complications strongly relate to psychological breastfeeding-related stress . The strength of the association between the presence of breast complications, breastfeeding-related stress, and exclusive breastfeeding practice may depend on participant characteristics and individual stress responses associated with breast complications. Some of the recommended management strategies for women who consider breastfeeding cessation because of breast complications, based on the breastfeeding pain reasoning model, include improving healing, improving the attachment of infant to the breast, maximizing comfortable positions for feeding, and psychological support . Self-management intervention for these strategies would be essential to improve breast complications and relieve breastfeeding-related stress .
As with previous studies, parity and education level were associated with exclusive breastfeeding in our study [4, 13]. However, the effect of parity on exclusive breastfeeding practice is not a simple correlation; rather, it often varies by study populations and previous breastfeeding experiences in multiparous women [5, 42, 43]. Women with higher education levels easily access health-related information and have more favorable attitudes toward breastfeeding [44, 45]. Such behavioral characteristics in women with higher education levels might help them achieve exclusive breastfeeding.
Planning to return to work by 6 months postpartum was associated with less exclusive breastfeeding at 3 months postpartum in our study. Postpartum women who planned to return to work in the earlier postpartum period tended to select partial breastfeeding or formula feeding [31, 46]. In Japan, continuing exclusive breastfeeding is difficult for some working women because of the work environment and nursery policies. A private space and consideration for expressing breast milk during working hours are often lacking, although many companies have been attempting to provide such environments. Nursery policies often refuse breast milk storage due to hygienic reasons. Such situations sometimes make women discontinue exclusive breastfeeding . In addition, partners’ attitudes against breastfeeding and childcare are possible important elements that prevent women from discontinuing breastfeeding after returning to work . Environmental and emotional support from family members, employers, and nursery staff members is essential for working women to continue exclusive breastfeeding.
Exclusive breastfeeding at 1 month postpartum is a strong predictor of the same at 3 months postpartum. However, even if women are not exclusively breastfeeding at 1 month postpartum, they may change to exclusive breastfeeding later on, as observed in the present study. The first 3 months after childbirth is a critical period for the establishment of exclusive breastfeeding . Thus, providing medical and psychological support is crucial during this period. Stress levels after breastfeeding, breastfeeding self-efficacy, and breast complications are possible key modifiable predictors. In Japan, women have state-funded medical checkups at 1 month postpartum. Thereafter, if they desire medical care regarding breastfeeding, they have to access it by themselves. By 1 month postpartum, healthcare providers’ advice and intervention for reducing breastfeeding-related stress responses, improving breastfeeding self-efficacy, and preventing breast complications may be effective in establishing and continuing exclusive breastfeeding. Further intervention studies are required to confirm the effectiveness.
The present study had two limitations. First, the dropout rate was higher than expected. Although no differences in infant feeding modality and salivary cortisol levels between dropouts and participants were found, the high dropout rate may have affected the relationship between cortisol levels and breastfeeding practice observed in the study. Second, we could not follow-up the participants for a full 6 months postpartum, although exclusive breastfeeding for 6 months is recommended. However, the key strength of this study is that we demonstrated the relationship between stress responses associated with breastfeeding and subsequent exclusive breastfeeding, using objective measures of stress.