Our aim was to investigate how age of achieving early motor developmental milestones differ among subjects with and without a history of parental psychosis and whether parental psychosis may alter the effects of the age of achievement on the risk of schizophrenia.

The study sample comprised 10,307 individuals from the prospective Northern Finland Birth Cohort 1966. A total of 139 (1.3%) cohort members suffered from schizophrenia by the age of 46 years. Out of them 19 (13.7%) had a parent with a history of psychosis, while among the non-psychotic cohort members this figure was 524 (5.2%).

Out of eight different motor milestones investigated, parental psychosis associated (p>0.05) with later learning of holding head up, grabbing object, and walking without support. In the parental psychosis group, significant risk factors for schizophrenia included later learning of holding head up and touching thumb with index finger. In the non-parental psychosis group risk estimates were lower and statistical significant milestones were different i.e. turning over, sitting without support, standing up, standing and walking without support. Interactions between parental psychosis and touching thumb with index finger and walking without support was found.

Although parental psychosis associated with delays in motor milestones in the first year of life, it does not explain the association between late achievement of motor milestones and later risk for schizophrenia