Is the season of birth association with psychosis due to seasonal variations in foetal growth or other related exposures? A cohort study.
OBJECTIVE
To investigate the association between season of birth and psychosis, and to assess whether any association is caused by seasonal fluctuations in foetal growth or other related exposures.


METHODS
Cohort of 747 432 Swedish males and females born between 1973 and 1980 and followed up from 16 years of age to 31 December 1999. Psychiatric admissions were identified using the Swedish Inpatient Discharge Register. The analysis is based on the 696 025 subjects with complete data.


RESULTS
A total of 506 (0.07%) subjects developed schizophrenia and 879 (0.13%) non-affective non-schizophrenic psychoses. There was a moderate increased risk of schizophrenia amongst winter births, hazard ratio 1.23 (95% confidence interval 0.96-1.59), but this did not reach conventional levels of statistical significance. There was no association with non-affective psychoses. We found no evidence that associations were confounded by measures of foetal growth or maternal socioeconomic position. There was no evidence that seasonal effects on schizophrenia differed in men and women.


CONCLUSIONS
Season of birth associations with schizophrenia do not appear to be confounded by birth-related exposures.
Fouskakis
D
D
Division of Psychiatry, Cotham House, Cotham Hill, Bristol, UK.
Gunnell
D
D
Rasmussen
F
F
Tynelius
P
P
Sipos
A
A
Harrison
G
G
D000293:Adolescent; D000328:Adult; D017668:Age of Onset; D001723:Birth Rate; D015331:Cohort Studies; D005314:Embryonic and Fetal Development / Q000502:physiology*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D011247:Pregnancy; D011297:Prenatal Exposure Delayed Effects; D015995:Prevalence; D011618:Psychotic Disorders / Q000453:epidemiology*; D012559:Schizophrenia / Q000453:epidemiology; D012621:Seasons; D012959:Socioeconomic Factors