Objective: Maternal filicide is not an isolated phenomenon. When a mother kills her child, she may be affected by many factors and confronted with different problems based on the child's developmental stage. In this study in Japan, a judicial sample of 96 adult women, convicted in their first trial for the murder or attempted murder of their children, was divided into four groups of mothers according to the age of the victim (25 women killed neonates, 22 women infants, 27 women preschool children, and 22 women schoolchildren and/or teenagers) in order to identify the factors that have a major impact on filicide in each group. Method: The socio-demographic, clinical, forensic, circumstantial, and offense characteristics, and legal disposition of 96 cases drawn from judicial records were compared among the four groups using the Kruskal-Wallis test; comparison of two groups was conducted using the Mann-Whitney test. Results: Neonaticide cases were distinguished from the other three groups by marked differences: a significantly higher rate of unmarried mothers, financial difficulties, absence of mental illness, and admission of not wanting an illegitimate child. In the other groups, mental disorders were frequent; in particular, post-partum depression was the primary cause of infanticide. For the two groups of cases involving a child older than one year, filicidal mothers were more affected by circumstantial factors such as health problems of the child or severe marital discord. These problems may then have caused a reactive mental disorder among these mothers. The risk of fatal abuse or neglect was higher for handicapped preschool children. Filicide-suicide was most frequently seen among school-aged children and/or teenagers who had serious behavioral problems, and these children often had a mental disorder. Conclusions: The classification of maternal filicide by age of the child demonstrated that there are specific issues for each group. Based on these findings, future directions for prevention include: appropriate sex education for youths to avoid unwanted pregnancy; organization of specialized mental health services for mothers with post-partum mental disorder; careful psychiatric risk assessment of mentally ill mothers; and development of diversified social support measures for child-bearing parents, especially those with identifiable financial or social difficulties.