In this epidemiologic study, we found that the risk of postpartum PTSD among exclusively breastfeeding mothers was substantially lower than among partially breastfeeding mothers at 42 days after childbirth, as reflected in either the total PCL-C score or the sub-clusters scores for re-experience, avoidance, and hyperarousal. This association remained after excluding women with a high EPDS score (greater than or equal to 13) at 3 days after childbirth and adjusting for several potential confounding factors, including family support, parity, mode of delivery, perceived birth trauma, and early contact / suckling as well as rooming-in.

The overall rate of postpartum PTSD in our study was 12.1%, which is within the range of 1–30% shown in a meta-analysis that included 15,637 women [3]. However, a survey involving 1136 Chinese mothers showed that the rate of PTSD was 6.1% at 6 to 8 weeks after childbirth, as assessed by the Perinatal Post-traumatic Stress Questionnaire [22]. This difference might be due to the different measurement instruments, as another Chinese study by Wang et al. [23] found a similar rate (11.6%) of postpartum PTSD evaluated by the PCL-C as is reported here. On the other hand, the discrepancy in the rate of PTSD could be attributed to differences in the duration of the follow-up periods. A longitudinal study that measured the rates of perinatal PTSD at three time points (pregnancy, 4 to 6 weeks postpartum, and 6 months postpartum) indicated that PTSD peaked at 4 to 6 weeks postpartum (11.9%) and subsequently declined at 6 months postpartum (9.2%) [18]. Thus, the higher PTSD rate in our study compared to the other Chinese study by Liu et al. [22] may be due to the different evaluation instruments and assessment times.

In our study, all mothers breastfed up to 42 days postpartum, benefiting from the promotion of breastfeeding in the study hospital in recent years. For instance, prenatal and postnatal breastfeeding education programs were provided to the perinatal women and their families by the obstetric team who were trained in the core curriculum for lactation consultant practice [24]. This study hospital has also been equipped with breastfeeding rooms under the launch of the “10 m2 of Love” campaign in accordance with local health administrative requirements [25]. However, the exclusive breastfeeding rate was still low in the present study. The results of our study sample are consistent with a 2018 national cross-sectional survey (n = 5237) in China, in which the partial breastfeeding and exclusive breastfeeding rates were 96.3% and 33.6%, respectively, within the first month postpartum [26].

Although no study to date has specifically assessed the association between exclusive breastfeeding and postpartum PTSD, previous studies suggested that breastfeeding might reduce the risk of mental health problems in postpartum women [5,6,7,8, 10]. In the present study, exclusive breastfeeding within 42 days after childbirth was associated with a reduced risk of postpartum PTSD, which may be mediated through the regulation of stress biomarkers such as cortisol [27,28,29]. One study has shown that women with PTSD had very high levels of cortisol in the postpartum period [30], raising the possibility that postpartum PTSD might be linked to the dysregulation of cortisol. Another study has found that lower cortisol levels after breastfeeding were associated with exclusive breastfeeding practices [31]. These observations may partially explain the association between exclusive breastfeeding and postpartum PTSD.

The association between exclusive breastfeeding and postpartum PTSD symptoms may also be mediated by oxytocin, which has been observed to provide therapeutic benefits to patients diagnosed with traumatic stress-related diseases, especially PTSD [32]. In response to suckling during breastfeeding, oxytocin released into the systemic circulation may play an important role in improving maternal mental health [33], accompanied by beneficial effects on mothers’ mood, affect, and stress, providing a calm and non-anxious state. Similar effects on emotion and stress as seen for breastfeeding have also been demonstrated in a randomized double-blind controlled trial administering oxytocin intranasally compared to a placebo [34], suggesting that the effectiveness of breastfeeding on maternal mood may be related to an increase in oxytocin levels.

To the best of our knowledge, this is the first study on the association between exclusive breastfeeding and postpartum PTSD. We excluded mothers with a history of or current psychiatric disorders including PTSD and postpartum depression at recruitment and ascertained only new PTSD cases. We also adjusted for potential confounding factors to minimize the impact of important covariates on the observed association between exclusive breastfeeding and PTSD.

This study also has certain limitations. First, the PCL-C has been frequently applied in the general population, but it has not been widely used in postpartum mothers. We are therefore unsure if it can ascertain all PTSD symptoms in our study participants. Second, study participants were recruited in a single center in China. Replication is needed to determine whether and to what extent findings from this study can be generalized to other populations. Third, since no infants were fed solely with formula in this study, we cannot explore possible associations between PTSD and formula feeding. Lastly, despite adjusting for confounding factors, residual confounding caused by unmeasured or unknown factors may still exist, given our limited knowledge on the mechanisms underlying the association between exclusive breastfeeding and PTSD.