This study examines the relationship between the psychological states of Korean mothers and early childhood development. In the study, 480 mothers were classified into three groups: cluster A mothers were happy and competent, cluster C were depressed and stressed, and cluster B were neither depressed nor happy. The tendency of infant developmental outcomes among the three clusters showed unexpected results. Infants in cluster C exhibited midlevel outcomes, whereas infants in cluster B showed the lowest development in all domains. These results showed that a mother's confusing and unclear feelings apparently have a negative effect on their infant's development. The findings of this study recommend that all new mothers, especially during the first year after birth, are to be supported with psychological education and counseling through screening, active monitoring, and initial treatment by health professionals.