BACKGROUND: Childcare facilities are a factor that lowers the established association of mother's postnatal psychiatric symptoms with children's behavioral problems. However, no studies have considered the prenatal psychiatric symptoms yet. This study examined whether the use of childcare facilities moderates the association of maternal psychological distress in early pregnancy and at two years postpartum with behavioral problems in children aged four years. METHODS: The present study was based on the data from 23,130 mother-child pairs participating in the Tohoku Medical Megabank Project Birth and Three-Generation Cohort Study. K6 was used to classify maternal psychological distress in early pregnancy and at two years postpartum into four categories: none in both prenatal and postnatal periods (none), only the prenatal period (prenatal only); only the postnatal period (postnatal only); both prenatal and postnatal periods (both). The children's behavioral problems were assessed using the Child Behavior Checklist for Ages 1½-5 (CBCL) aged four years. The clinical range of the externalizing, internalizing, and total problem scales of the CBCL was defined as having behavioral problems. To examine whether availing childcare facilities moderates the association between maternal psychological distress and children's behavioral problems, we conducted a stratified analysis based on the use of childcare facilities or not, at two years of age. The interaction term between maternal psychological distress and use of childcare facilities was included as a covariate in the multivariate logistic regression analysis to confirm the p-value for the interaction. RESULTS: The prevalence of the clinical ranges of externalizing problems, internalizing problems, and clinical range of total problems were 13.7%, 15.4%, and 5.8%, respectively. The association of maternal psychological distress with a high risk of children's behavioral problems was significant; however, the association between prenatal only psychological distress and externalizing problems in the group that did not use childcare facilities was not significant. Interactions between the use of childcare facilities and maternal psychological distress on behavioral problems in children were not significant. CONCLUSIONS: Use of childcare facilities did not moderate the association of maternal psychological distress in early pregnancy and at two years postpartum with behavioral problems in children aged four years.