Abstract Background The coronavirus disease 2019 (COVID-19) pandemic has recently become the most important issue in the world. Very few reports in Japan have examined the impact of the COVID-19 pandemic on peripartum mental health. We examined the status of postpartum mental health before and during COVID-19 pandemic from a consecutive database in a metropolitan area of Japan. Methods The subjects were women who had completed a maternity health check-up at a core regional hospital in Yokohama during the period from April 1, 2017, to December 31, 2020. We collected the subjects’ scores for the Edinburgh Postnatal Depression Scale (EPDS) and the Mother-to-Infant Bonding Scale (MIBS) at 1 month postpartum. The subjects were divided into four groups (three Before COVID-19 groups and a During COVID-19 group). MANOVA and post-hoc tests were used to determine mental health changes in the postpartum period among the four groups. Results The Before and During COVID-19 groups contained 2844 and 1095 mothers, respectively. There were no significant difference in the total scores of the EPDS and MIBS among the four groups. However, the EPDS items related to anxiety factors were significantly higher and the EPDS items related to anhedonia and depression factors (excluding thoughts of self-harm) were significantly lower in the During COVID-19 group. Conclusion The EPDS scores changed in connection with the COVID-19 pandemic. Anxiety, which represent hypervigilance, was significantly higher and anhedonia and depression were significantly lower in the During COVID-19 group. Our results may reflect COVID-19-related health concerns and a lack of social support caused by the COVID-19 pandemic.