Seasonal fluctuation in birthweight in schizophrenia

The birthweights of 450 schizophrenic patients born in 1971–1978 were compared with those of individually matched controls from the same birth cohort. Schizophrenics born in the second quarter of the year (April to June) had a significantly lower birthweight than their controls and this was not attributable to differences in the duration of gestation. Analogous proband/control differences were not found in 301 patients with affective psychoses, but there was a similar, much smaller, seasonal fluctuation in birthweight in the general population. These findings may be a clue both to the genesis of the “season of birth effect” and to the identity of the intrauterine influences contributing to the aetiology of schizophrenia. Folate deficiency may be implicated.