Lower salivary cortisol levels after breastfeeding at 1 month postpartum were associated with exclusive breastfeeding at 3 months postpartum, regardless of parity. Furthermore, 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 between cortisol levels and exclusive breastfeeding at 1 month postpartum. Cortisol levels usually decrease after breastfeeding since oxytocin inhibits cortisol secretion in women without mood distress [23]. Nevertheless, more than 40% of participants in the present study had increased cortisol levels after breastfeeding compared with 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 decreased in women with psychological stress compared to those without [11, 24]. Thus, it is assumed that breastfeeding-related stress affects subsequent exclusive breastfeeding practice through psychological burden and associated behavioral changes, 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 preventing breast complications, relieving anxiety by providing advice about breastfeeding concerns, and relaxation during breastfeeding could be effective [25]. In addition, maternal cortisol responses to breastfeeding vary according to the function of the CD38 rs3796863 [26], an ectoenzyme that mediates the release of oxytocin. A recent study has indicated that the CD38 rs3796863 CC genotype is associated with a reduced release of oxytocin during breastfeeding and, accordingly, fewer cortisol-reducing responses to breastfeeding [27]. 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 the analyses without considering the gene. Further studies are needed to clarify the associations.
A higher BSES-SF score was associated with subsequent exclusive breastfeeding as reported by a previous study of another population [28]. 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” [9, 17]. Women with positive answers to such items seemed less stressed about breastfeeding. Women with lower breastfeeding self-efficacy were also reportedly more likely to perceive milk insufficiency [9, 29]. Such a perception has been described as a factor related to cessation of exclusive breastfeeding [30]. 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 may 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 [31]. This was because breastfeeding experiences in primiparas were affected more by subjective norms and social environment than breastfeeding self-efficacy [31]. Therefore, the effectiveness of improving breastfeeding self-efficacy on breastfeeding practices may differ according to parity. A systematic review showed that prenatal and postpartum interventions focusing on improving breastfeeding self-efficacy lead to exclusive breastfeeding [32]. However, in Japan, the effects of prenatal intervention using a breastfeeding self-efficacy workbook are limited and effective in only baby-friendly hospitals and in the early postpartum period [33]. Development of more effective approaches taking parity into consideration are needed for postpartum Japanese women.
In multiparas, absence of breast complications at 1 month postpartum was associated with subsequent exclusive breastfeeding. Although this association supported previous studies [34, 35], we did not ascertain the reason why it was observed only in multiparas. In general, primiparas experienced more breast complications than multiparas [36], as observed in the present study. Furthermore, breast complications strongly relate to psychological breastfeeding-related stress [37]. 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. However, educational intervention for prevention of breast complications is essential to relieve breastfeeding-related stress and increase exclusive breastfeeding practices.
As with previous studies, parity and education level were associated with exclusive breastfeeding [4, 12]. 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, 38, 39]. Women with higher education levels easily access health-related information and have more favorable attitudes toward breastfeeding [40, 41]. Such behavioral characteristics in women with higher education levels might help them achieve exclusive breastfeeding.
Returning to work by 6 months postpartum was associated with less exclusive breastfeeding at 3 months postpartum. Postpartum women who planned to return to work in the earlier postpartum period tended to select partial breastfeeding or formula feeding [29, 42]. 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 [43]. In addition, partners’ attitudes against breastfeeding and childcare could be important elements that prevent women from discontinuing breastfeeding after returning to work [44]. Environmental and emotional support from family members, employers, and nursery staff members may be 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 as observed in the present study. The first 3 months after childbirth remain a critical period for the establishment of exclusive breastfeeding [45]. Thus, medical and psychological approaches in this period would be significant. 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. Second, we could not follow 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 showed the relationship between stress responses associated with breastfeeding and subsequent exclusive breastfeeding, using objective measures of stress.