BackgroundA negative effect of paternal depression on child development has been revealed in several previous studies. The aims of this study were to examine the prevalence and relevant factors associated with paternal postnatal depression at four months postpartum, including age, part-time work or unemployment, experience of visiting a medical institution due to a mental health problem, economic anxiety, unexpected pregnancy, pregnancy with infertility treatment, first child, partner’s depression, and lower marital relationship satisfaction.MethodsWe distributed 2032 self-report questionnaires to couples (one mother and one father) with a 4-month old infant between January and April 2013. Data from 807 couples (39.7 %) were analyzed. Depressive symptoms were measured with the Edinburgh Postnatal Depression Scale (EPDS). In order to clarify the factors related with paternal depression, a logistic regression analysis was conducted.ResultsOne hundred and ten fathers (13.6 %) and 83 mothers (10.3 %) were depressed. According to the logistic regression analysis, paternal depression was positively associated with partner’s depression (adjusted odds ratio (AOR) 1.91, 95 % confidence interval (CI) 1.05–3.47), and negatively with marital relationship satisfaction (AOR 0.83, 95 % CI 0.77–0.89). History of infertility treatment (AOR 2.37, 95 % CI 1.32–4.24), experience of visiting a medical institution due to a mental health problem (AOR 4.56, 95 % CI 2.06–10.08), and economic anxiety (AOR 2.15, 95 % CI 1.34–3.45) were also correlated with paternal depression.ConclusionsThis study showed that the prevalence of paternal depression at four months after childbirth was 13.6 % in Japan. The presence of partner’s depression and low marital relationship satisfaction were significantly correlated with paternal postpartum depression, suggesting that health professionals need to pay attention to the mental status of both fathers and mothers, and to their relationship.