Seasonality of birth in seasonal affective disorder

Season of birth or seasonal changes in putative etiologic factors are thought to influence the development of several psychiatric illnesses. The aim of this investigation was to examine seasonal differences in the frequency of birth in a clinical sample of patients with seasonal affective disorder (SAD).

553 outpatients suffering from SAD-DSM-IV-defined depressive disorder with winter-type seasonal pattern-who had been diagnosed and treated at the Department of General Psychiatry (University of Vienna, Austria) between 1994 and 2003, were included in this evaluation. We compared the observed number of births in our sample with expected values calculated from the general population.

There was a significant deviation of the observed number of births from the expected values calculated on a monthly basis (p = .009). When comparing quarters (periods of 3 months), we found fewer births than expected in the first quarter of the year and a slight excess of births in the second and third quarters (p = .034). There were also more births in the spring/summer season and fewer than expected in fall and winter (p = .029). Interestingly, patients with melancholic depression were more frequently born in fall/winter and less often in spring/summer compared with patients with atypical depression (p = .008).

Besides genetic factors, season of birth or seasonal changes in environmental factors also could influence the development of SAD. In addition, birth effects seem to be dependent on the symptom profile of the patients, but further studies are needed to elucidate the underlying mechanisms of these observations.