Abstract A substantial minority of children are born as a consequence of an unplanned pregnancy. Yet little is known about the impact of unplanned/unwanted pregnancy (UP) on long-term health outcomes for children. This study aimed to examine the association between UP and child mental health and behavioural problems at 14 years, and whether this association is confounded or mediated by other variables. Data were from a pre-birth prospective study that included 4765 mothers and their children (48.4% female and 51.6% male) followed up from pregnancy to 14 years of the child's age in Brisbane, Australia. Child anxiety/depression, aggression, delinquency, attention problems, withdrawal problems, somatic complaints, social problems, thought problems, internalizing, externalizing and total problems were measured using the Achenbach's Youth Self Report at 14 years. Child smoking and alcohol consumption were self-reported at 14 years. UP was prospectively assessed at the first antenatal visit of pregnancy. UP as reported by mothers at first antenatal visit predicted elevated levels of problem behaviours and increased substance use in children at 14 years. The impact of UP on child mental health and problem behaviours is partly due to the confounding effect of other variables, such as maternal socio-demographic status, mental health and substance use during pregnancy. Further research is needed to investigate the mechanism of association between UP and child aggression and early alcohol consumption at 14 years.