Factors influencing postpartum depression among Japanese parents: A prospective longitudinal study

Abstract Aim Postpartum depression (PPD) may have negative effects on the parents and lead to impaired cognitive, socioemotional, and behavioral development in their children. The purpose of this study was to examine factors associated with PPD in parents during the first year after delivery. Methods This study used a self‐administered questionnaire. Questionnaires were mailed at 5 days, 3 months, 6 months, and 1 year after delivery, respectively. The particpants were 107 pairs of mothers and fathers. PPD was assessed using the Edinburgh Postnatal Depression Scale (EPDS). Data on sense of coherence (SOC), Quality Marriage Index, Social Support Scale, Mother‐to‐Infant Bonding Scale, and sociodemographic variables were collected. Multiple regression analysis was performed to examine the strength of the association between several variables and the EPDS at each survey period for fathers and mothers, respectively. Results The prevalence of PPD in the first‐year postpartum ranged from 12.1%–23.4% to 7.5%–8.4% for fathers and mothers, respectively. SOC had the strongest impact on EPDS scores for both fathers and mothers at all four survey periods. Conclusions Our findings suggest that stress coping skills are an important factor affecting PPD throughout the first‐year postpartum for both fathers and mothers.

do children of mothers with clinical depression. 9,10 Therefore, PPD is a global public health concern 11 and also a key issue in the "Healthy Parents and Children 21 (2nd Phase)" initiative being promoted by the Japanese Ministry of Health, Labor and Welfare (MHLW). 12 Implementing a targeted and effective strategy requires understanding of factors that influence PPD. Several studies have identified various risk factors for maternal PPD. 2,[13][14][15] The available evidence on risk factors associated with PPD in fathers is also growing. 16,17 While the previous study on PPD has identified various factors, sorting out modifiable factors, including psychosocial factors, may help improve postpartum outcomes for parents. For instance, sense of coherence (SOC) has been reported as a risk factor for PPD, but most reports have focused on mothers, [18][19][20][21] and few have included fathers. 22,23 Mother-infant bonding may affect maternal mental health after childbirth. 24,25 There is also growing evidence available on risk factors associated with PPD in fathers (e.g., unemployment, low social support, etc.). 16,17 However, given the evidence of an association between paternal and maternal depressive symptoms during pregnancy and after delivery, 26 it is not sufficient to identify either paternal or maternal factors. 16 Longitudinal studies investigating factors associated with PPD involving both parents are needed to construct effective preventive strategies. Previous longitudinal studies that included both parents have shown that mother-in-law and daughter-in-law relationship satisfaction, mutual depression, and marital relationship satisfaction are related to PPD. 27,28 A previous study of married couples in Japan reported that history of mental health disorders, household income, infant disease under medical treatment, partner depression, and marital satisfaction were associated with PPD in fathers. 17,29 However, their study conducted in Japan investigated only one (after 4 months) or two (after 1 month and 6 months) time periods between 1 and 6 months postpartum. Considering the possibility that PPD may occur during the first year after delivery, a longitudinal investigation of at least 1 year postpartum is warranted.
Longitudinal studies including fathers and mothers simultaneously on factors affecting PPD are limited, especially in the Japanese context. Furthermore, previous studies that have longitudinally examined factors affecting PPD indicate that these factors may vary by postpartum time. Therefore, this study aimed to conduct a longitudinal survey from a few days to 1 year after delivery to investigate factors affecting PPD in fathers and mothers at each survey period (a few days, 3 months, 6 months, and 1 year after delivery). We also investigated whether psychosocial factors in the few days postpartum predicted PPD in subsequent periods.

| Participants and procedure
This prospective longitudinal study investigated factors related to PPD in parents from a few days to 1 year after delivery. Between September 2011 and February 2013, 1276 couples who had given birth at a local obstetric clinic in urban Kyushu, Japan, were enrolled. The clinic was staffed by five full-time obstetricians, several part-time obstetricians, and 25 nursing personnel (midwives, nurses, assistant nurses, and nursing assistants), who played a central role in the development of the local child delivery environment. Through collaboration with a perinatal medical center that provides advanced and specialized medical care, the clinic offers a safe perinatal care system for women with high-risk pregnancies. The clinic also offers 24-hour telephone consultation to expectant and postpartum mothers, health checkups, and telephone follow-ups at 2 weeks and 1 month postpartum, classes on breast massage and nipple care for postpartum mothers, and access to a postpartum care center (established in 2014). We distributed self-report questionnaire to couples who gave consent at a few days (T1), 3 months (T2), 6 months (T3), and 1 year (T4) after delivery, respectively. The questionnaire included an enclosed reply envelope; thus, the responses were collected via mail. The questionnaire explained to participants the purpose of the study and that the information collected would be kept confidential and used only for the purposes of this study. By answering the questions in the questionnaire, the respondents reaffirmed their consent to the study. Our study protocol was approved by the Saga University (Faculty of Medicine) Ethics Committee (Ref. Nos. 23-60).

| Postpartum depression symptoms
PPD was measured using the Japanese version of the Edinburgh Postnatal Depression Scale (EPDS). 30,31 The EPDS is a self-rated questionnaire designed to screen for symptoms of emotional distress during pregnancy and postpartum period, 30 with validity and reliability verified for the Japanese version. 31 The scale consists of 10 items, each graded on a scale from 0 to 3, with a total score range of 0-30. Mothers who score 9 or higher are considered to have depressive symptoms, and this cutoff score showed a sensitivity of 75% and a specificity of 93%. 31 Whereas fathers with scores of 8 or higher are considered to have depressive symptoms. 17 Since it is generally believed that men may be less expressive about their feelings, a lower cutoff score is applied than the cutoff score used for mothers. 32 The Cronbach's α of the EPDS for each period in the current sample was 0.71-0.82 and 0.75-0.83 for fathers and mothers, respectively.

| Sense of coherence
We used the Japanese version of the SOC questionnaire, which consists of the 13 items. 33 Participants rated their level of agreement with each question on a 7-point Likert scale, with a total score ranging from 13 to 91. Higher total scores reflect stronger SOC. A previous study validated the Japanese version of the scale. 34 Permission to use SOC is granted through the Society for Theory and Research on Salutogenesis (STARS). 35 The Cronbach's α of the SOC for each period in the current sample was 0.85-0.90 and 0.88-0.91 for fathers and mothers, respectively.

| Marital satisfaction
Marital satisfaction was assessed using the Quality Marriage Index (QMI). 36,37 The questionnaire consists of the six items, and participants rate their level of agreement with each item on a 4-point Likert scale. A high QMI score indicates that the marriage partnership is viewed favorably. The Cronbach's α of the QMI for each period in the current sample was 0.87-0.94 and 0.90-0.96 for fathers and mothers, respectively.

| Social support scale (SSS)
We assessed social support for fathers at the workplace using the subscales of the Brief Job Stress Questionnaire (BJSQ). 38 The questionnaire was developed by the MHLW and is widely used for stress check program in Japanese companies. 39 The BJSQ consists of 57 items, 9 of which measure social support, including support from superiors, co-workers, family, and friends. Participants rate their level of agreement with each item on a 4-point Likert scale.
Higher total scores reflect lower levels of support. The reliability and validity of the BJSQ have been verified. 40 The Cronbach's α of the social support scale for each period in the current sample was 0.85-0.87.

| Maternal-infant bonding
The Mother-to-Infant Bonding Scale (MIBS) was used to assess bonding between mother and infant. 41 This scale was originally developed as a 9-item questionnaire 41 and was modified to 10 items in the Japanese version. 42 Participants are asked to rate the each items on a 4-point Likert scale from 0 to 3. The higher scores reflect worse mother-to-infant bonding. The Cronbach's α was 0.57-0.70 for the current study.

| Assessment scale at each survey period and their relationship to the EPDS
SSS in fathers and MIBS in mothers also differed significantly according to the survey period (SSS, p < 0.001; MIBS, p = 0.004). SSS in fathers was significantly lower at T1 than at T2 (p = 0.001), T3 (p = 0.001), and T4 (p < 0.001). MIBS in mothers was higher at T1 than at T2 (p = 0.021) and T3 (p = 0.043). Table 4 shows the correlations between EPDS scores and each of the assessment score for fathers and mothers, respectively. In fathers, SOC and QMI were significantly correlated with EPDS at all survey periods. SSS was significantly correlated with EPDS at T2, T3, and T4. In mothers, SOC and MIBS were significantly correlated with EPDS at all survey periods. QMI was significantly correlated with EPDS at T2, T3, and T4.

TA B L E 3
Comparison of each assessment scale by survey period. T1  T2  T3  T4  T1  T2  T3  Note: Pearson's correlation analysis *p < 0.05, **p < 0.01, ***p < 0.001. Table 6 shows the association between the values of the rating scale measured at T1 and the EPDS measured after that. EPDS at T2, T3, and T4 in fathers was significantly associated with SOC at T1.

Father's EPDS Mother's EPDS
Conversely, mothers' EPDS at T2 and T3 were significantly associated with SOC at T1.

| DISCUSS ION
This longitudinal study found that 12.1%-23.4% for fathers and 7.5%-8.4% for mothers showed signs of PPD during the first year after delivery. Furthermore, SOC was associated with EPDS scores for both fathers and mothers at all survey periods.
The prevalence of paternal PPD at 6 and 12 months in this study was slightly higher than in other Japanese studies. 17,29 According to a meta-analysis of the prevalence of paternal PPD in Japan, most studies using the EPDS were conducted within the 6-month postpartum period. 5 That meta-analysis suggests that the prevalence of paternal PPD peaks at 3-6 months after delivery. In a Japanese longitudinal study using the Center for Epidemiologic Studies Depression Scale, the highest prevalence of paternal PPD was at 1 year postpartum. 43 Our findings suggest that PPD is more common in fathers and that we need to pay particular attention to the occurrence of paternal PPD during the 6 months to  45 While life events such as pregnancy and childbirth are joyful and happy for many fathers and mothers, these are also stressors. 46 During the transition to parenthood, stress levels increase as parents face major changes in roles, relationships, and lifestyles. 47,48 Successfully coping with those stressors is important to prevent PPD. SOC has also been reported to increase an individual's effectiveness in coping with stressors 21 and is an important factor in preventing postpartum emotional disturbances.
Furthermore, previous study not related to childbirth indicate that interventions may be able to increase SOC. 49 Our findings suggest TA B L E 5 Results of multiple regression analysis for father's and mother's EPDS. Mother's EPDS   T1  T2  T3  T4  T1  T2  T3  T4   β  β  β  β  β  β  Note: Multiple regression analysis (forced entry method) using father's and mother's EPDS as dependent variables.*p < 0.05, **p < 0.01. that developing strategies that enhance SOC may play a vital role in managing PPD for both fathers and mothers.

Father's EPDS
Although the period of influence on EPDS was limited, QMI for both fathers and mothers, MIBS for mothers, and SSS for fathers were found to be associated with EPDS. Previous studies have shown that marital satisfaction is associated with PPD for both fathers and mothers. 27,50 It has also been found that mother-infant bonding is a predictor of psychological distress in mothers 24 and that social support is related to paternal PPD. 16 However, given the bidirectional association between these psychosocial factors and postpartum depression, the relationship may be more complex. For instance, disturbances in mother-infant bonding in the first year postpartum are also predicted from having PPD. 25 Our findings suggest that factors related to PPD may differ depending on the postpartum period, underscoring the need to sort out the complexity of these relationships according to period.
Several limitations should be considered in interpreting our results. First, we included couples in which both mothers and fathers completed questionnaires at all four time points, hence the low response rate. Assuming that parents who understood the purpose of this study and had the mental and financial resources to respond with a high awareness of childcare, the EPDS for non-respondents could be high. However, future research is needed to clarify this.
The family environment, including mental and financial concerns, is also crucial for PPD. 29 In addition, since the present study was conducted in a single clinic located in an urban area, the findings need to be validated in future studies with a larger and more diverse sample. Second, we assessed PPD based on a self-reported EPDS score rather than a full diagnostic procedure. However, since the EPDS is a validated and reliable instrument that is frequently used worldwide, the risk of misclassification bias is expected to be small. Third, residual confounders may have occurred from unmeasured confounders such as sociodemographic variables including income and educational history and parental mental status during the pregnancy. Finally, the present study was conducted using data obtained before the COVID-19 pandemic. Social restrictions and loss of social support associated with COVID-19 may be associated with PPD. 51 Women with perinatal psychological problems and those who experienced emotional support may be positively affected by less external stimuli due to lockdown restrictions. 52 Therefore, the social context of COVID-19 should be considered when interpreting our results. Additionally, the situation surrounding maternal and child health care has been pointed out as declining birthrates, late marriages and childbearing, rising unmarried rates, the shift to nuclear families, isolation of childcare, and child abuse. 12 Systems have been developed to enable parents to take childcare leave in cooperation. 44 It is necessary to target support not only to mothers and their children but also to parents and their children, including fathers. In particular, support for fathers is also essential, as we live in an era where men are required to take parental leave. Despite these limitations, this is a significant one-year longitudinal study investigating psychosocial factors for parents and underscores the need for interventions for psychosocial factors, including SOC just after birth and at each point. Note: Multiple regression analysis (forced entry method) using father's and mother's EPDS as dependent variables. *p < 0.05, **p < 0.01.

| CON CLUS IONS
This longitudinal study was conducted from a few days to 1 year after delivery to investigate the factors affecting PPD in fathers and mothers during each survey period. Our findings suggest that SOC is the strongest predictor of PPD for both fathers and mothers throughout the first year after delivery. Developing strategies to enhance stress coping skills may be important to prevent PPD in both fathers and mothers.

ACK N OWLED G M ENTS
We would like to express our heartfelt gratitude to all the people who cooperated in this study and the late Professor Hideyuki Nakane for his extensive guidance.

FU N D I N G I N FO R M ATI O N
This study was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (22592485).

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are not publicly available due to ethical restrictions. Specifically, informed consent for public data release was not obtained from the participants.

E TH I C S S TATEM ENT
Approval of the research protocol by an institutional reviewer board: The study protocol was approved by the Saga University (Faculty of Medicine) Ethics Committee (Ref. Nos. 23-60).
Informed consent: All informed consent was obtained from the participants.
Registry and the registration no. of the study/trial: N/A.
Animal studies: N/A.