ObjectivesMothers with young children are particularly vulnerable to the impacts of the lifestyle changes brought about by the COVID-19 pandemic. However, the association between such changes and maternal mental health has not been examined, and comparable pre-COVID-19 baseline data were lacking. Thus, we aimed to examine the relationships between changes due to COVID-19 pandemic and the development of depressive and anxiety symptoms among mothers of infants and/or preschoolers in Japan.DesignProspective follow-up study. The baseline survey was conducted in February 2020, and the follow-up survey was conducted in June 2020.SettingAll 47 prefectures in Japan.ParticipantsAt the baseline, 4700 mothers of infants and/or preschoolers (0–6 years) participated in the online survey (100 respondents per prefecture); 2489 of them also participated in the follow-up survey. After excluding 203 participants with a higher risk of severe mental illness at the baseline, 2286 were included in the analysis.Outcome measuresThe Kessler Psychological Distress Scale was used to measure depressive and anxiety symptoms, with a cut-off point of 13 or more. We estimated the adjusted OR (AOR) using multiple logistic regression analysis.ResultsDuring the follow-up period, 151 (6.6%) of respondents newly developed depressive and anxiety symptoms. Participants who experienced a shortage of relaxation time (AOR 1.61, 95% CI 1.06 to 2.47), increased difficulty in child rearing (AOR 1.89, 95% CI 1.32 to 2.70), increased partner aggression (AOR 2.93, 95% CI 1.42 to 6.05) and an increased sense of unfairness (AOR 1.74, 95% CI 1.10 to 2.73) were more likely to develop these symptoms.ConclusionsChanges in circumstances and perceptions during COVID-19 outbreak were significantly related to the development of depressive and anxiety symptoms among mothers of young children. Strategies to reduce solo parenting and increase social awareness related to domestic violence are needed.