Background <p>Few studies have investigated the predictors of short and long interpregnancy intervals in Japan. Thus, this study investigated the maternal sociodemographic characteristics of short and long interpregnancy intervals among multiparous women using nationwide data in Japan.</p> Methods <p>The data of the Longitudinal Survey of Newborns in the 21st Century (2010 Cohort) in Japan were used, and the interpregnancy intervals of mothers were calculated. The age group, marital status, number of births, employment status, educational attainment of mothers, and household income were used as explanatory variables. Interpregnancy intervals of less than 12 months and 60 months or more were defined as short and long interpregnancy intervals, respectively. A modified Poisson regression model was employed to investigate the relationships between short and long interpregnancy intervals and maternal characteristics, and unadjusted and adjusted analyses were conducted.</p> Results <p>A total of 19,879 infants were used in the analysis. The results of the adjusted regression analysis showed that the risk ratio for short interpregnancy intervals increased with a decrease in maternal age, whereas the opposite trend was observed for long interpregnancy intervals. In addition, low educational attainment was positively associated with long interpregnancy intervals, and low household income was positively associated with short interpregnancy intervals. Employment status also played a role, with working mothers showing a negative association with short interpregnancy intervals and a positive association with long interpregnancy intervals.</p> Conclusions <p>This study showed that maternal sociodemographic characteristics such as employment status, educational attainment, and household income were associated with short and long interpregnancy intervals in Japan.</p>