Objectives To measure the extent and analyse the mechanisms of underestimation of neonaticides (infanticides in the first 24 h of life) and to identify characteristics of neonaticidal mothers.

Design A retrospective study was carried out in 26 courts in three French regions covering 34.6% of all births in France. Included in the study were all cases of infants dying on their day of birth during a 5-year period (1996–2000) that were submitted to courts by the state prosecutor. Complete court case data cover 1996–2007. WHO-International Classification of Diseases cause of death codes assigned in the mortality statistics were compared to causes assigned by the courts. Analysis was carried out on the psychiatric assessments of the mothers.

Results 27 cases of neonaticides were analysed. 17 mothers were identified. We observed sizeable underestimation of neonaticides in official mortality statistics (0.39 per 100 000 births vs 2.1 in our study). Mothers' median age was 26 years; one third had at least three children. More than half lived with the child's father. Two thirds were employed and did not differ significantly from women in the general population regarding occupation. These women appeared to have low self-esteem, be immature and dependent, but did not have frank mental illness. There was no true case of ‘denial of pregnancy’. Contraception was not used.

Conclusion This study has implications for prevention in terms of contraception and appropriate targeting of vulnerable women. It encourages the development of case–control studies on maternal risk factors within the framework of the French birth cohort currently being established.