Background This study investigated whether ‘unwanted pregnancy’ (i.e. a negative or ambivalent attitude towards the pregnancy/reproduction) is associated with schizophrenia-spectrum and affective disorders in the offspring in adulthood, and if so, whether other pregnancy, perinatal, childhood or genetic-risk factors account for this association.Method In a prospective study beginning during pregnancy, unwanted pregnancy (in combination with other early life risk factors) was studied in relation to adult mental disorders in 75 genetic high-risk (HR) and 91 normal-risk (NR) offspring, defined through maternal psychosis history. Early life risk factors were studied through personal interviews, observations and medical records, and offspring mental disorders were independently diagnosed through follow-up examination at about 22 years of age.Results Unwanted pregnancy by itself was significantly related to adult offspring schizophrenia-spectrum disorders in both the total sample and the HR subgroup, but the effect was found to be limited to the HR group and occurred in interaction with genetic risk. Other co-temporaneous pregnancy stressors and later perinatal complications, malformations and early childhood environmental stressors could not explain this relationship. Unwanted pregnancy also interacted with genetic-risk status in relating to affective disorders in the offspring.Conclusions Unwanted pregnancy, when occurring together with genetic risk for psychosis, was found to be related to both adult schizophrenia-spectrum and affective mental disorders in the offspring. Although the effect of unwanted pregnancy could be mediated by other yet-unidentified factors, unwanted pregnancy might be a functional, discrete environmental psychosocial factor with its own deleterious impact on offspring mental development, when co-occurring with genetic risk.