We investigated the risk factors (especially preterm birth) for postpartum depressive symptoms in mothers who visited the clinic for medical checkup at 1 month postpartum using EPDS. Consequently, the chi-square test and multivariate logistic regression analyses revealed that “preterm birth” was associated with postpartum depressive symptoms in the first postpartum month in Japan.
In the present study, preterm births occurred at 35 to 36 weeks of gestation and the results supported a previous study showing that late preterm birth was a risk factor for postpartum depressive symptoms at 1 month postpartum . The National Center for Child Health and Development stated that “late preterm birth is defined as birth between 34 and 36 weeks of gestation, and late preterm infants are said to have many health issues” . Late preterm infants were revealed to be physiologically and metabolically immature and often to have breast-feeding problems [30–32]. Many mothers of preterm infants have been found to have high anxiety about the health of their children, and post-traumatic symptoms due to shock from unexpected preterm birth, inadequate psychological preparation for motherhood, inadequate preparation of the home and other environments, and experiences different from those expected or anticipated for birth [23, 24]. These findings suggest that mothers of late preterm infants are routinely in a state of stress. It is believed that routine stress is one of the risk factors for postpartum depression and that the association between preterm birth and postpartum depressive symptoms is related to early stress of parents and problems with the interaction between mother and child [17, 22]. In addition, from the observation in the present study, it is believed that the association between late preterm birth and postpartum depressive symptoms depend on routine stress and negative feelings of mothers toward children. In contrast, some studies showed no association between preterm birth and postpartum depressive symptoms. De Paula Eduardo et al.  concluded that the reasons for varying results might include differences in the confounding variables used, in the tools used for screening and evaluating depression, and in the study subjects . Further studies are needed to gain a better understanding of the association between late preterm birth and postpartum depressive symptoms.
The present study showed that “primipara”, “history of psychiatric disease” and “male infant” were significantly associated with postpartum depressive symptoms at the medical checkup at 1 month postpartum. Regarding parity, primipara were found to be more prone to postpartum depressive symptoms when compared to multipara. Previous studies on the relationship between the birth experience and postpartum depression reported that primipara had higher EPDS scores and were more prone to postpartum depression when compared to multipara [1, 18, 34–36]. The results from the present study supported those from the previous studies. It has been reported that a greater proportion of primipara had suspected postpartum depression compared to multipara, primipara were under high stress particularly in the early postpartum period according to the Parental Stress Scale, and that primipara had higher anxiety and depression scores on the Hospital Anxiety and Depression Scale compared to multipara [35–38]. Nakano et al. considered that women who experienced depression after the first childbirth had low motivation for a second childbirth and did not want a further pregnancy, resulting in a higher frequency of postpartum depression in primipara compared to multipara . We believe that these reasons also explain the results of the present study showing higher frequency of postpartum depressive symptoms in primipara compared to multipara. Conversely, Dietz et al. reported that there was no significant difference in the frequency of postpartum depression between primipara and multipara . Their results contradict the results obtained in the present study. We believed that the reasons for this difference might include differences in methods, and the timing of and tools for evaluating postpartum depressive symptoms. Furthermore, as Takegata et al. have shown, varying results may also be due to differences in the environment, such as differences in societies and cultures .
The present study also showed that women with a history of psychiatric disease were more likely to have postpartum depressive symptoms than women without a history of psychiatric disease. Previous studies have revealed that a history of psychiatric disease was one of the risk factors for postpartum depression [8, 13, 16, 40]. The greatest risk factor for postpartum depression was shown to be symptoms of depression during pregnancy, and a risk factor for depression during pregnancy was shown to be a prior history of major depressive disorder . A prior history of major depressive disorder is thus considered to be a risk factor for postpartum depressive symptoms. In addition, it was demonstrated that a greater proportion of women with postpartum depression had a family history of postpartum depression than those without postpartum depression, and the predictive factors for recurrent depression in elderly people included a high level of neuroticism and a low level of mastery [41, 42]. Therefore, we believed that in the present study, women with a history of psychiatric disease were more likely to have symptoms of postpartum depressive due to susceptibilities related to genetic factors and personality traits.
In addition, the present study showed that mothers with male infants were more prone to postpartum depressive symptoms than mothers with female infants. A study by Mori et al. in Japan and other studies in other countries also showed that the infant’s expected sex was associated with postpartum depression at 1 month postpartum [1, 25, 43]. On the other hand, it was reported that in India, Nigeria and China, the birth of female infants was associated with postpartum depression because the husbands and family members believed that male infants were needed to continue the family line and develop the family business leading to a strong desire for the birth of male infants, and hence the birth of female infants might negatively affect the marital relationship [25, 43, 44]. Takegata et al. reported that currently, parents in Japan have a preference for female infants because parents’ expectations of better relationships with biological daughters and of biological daughters caring for them in old age outweigh their desires for a family heir or for future financial support . In addition, it was reported that sex preference could cause postpartum depression when there was a disagreement between the mother and her mother-in-law in that regard . In the present study, the birth of male infants might have been associated with postpartum depressive symptoms due to a similar cause.
There are several limitations to the present study. First of all, in general, it has been shown that people underreport undesirable behaviors. The present study used a self-administered questionnaire, the subjects’ responses of which might have been influenced by their perceptions of the social desirability of their responses . Secondly, the results of the present study are difficult to generalize, since the target population consisted of the patients of a single hospital. Finally, since the hospitals covered by the present study were practitioners, high-risk pregnant women were transferred to larger hospitals. As a result, the number of subjects with factors affecting the EPDS might have been reduced. In the future, it will be necessary to increase the number of hospitals included and conduct longitudinal research.