Season of birth and neurodevelopmental disorders: summer birth is associated with dyslexia.

OBJECTIVE Increased risk for certain psychiatric disorders has been associated with season of birth. This study was undertaken to look for hypothesized season-of-birth effects for dyslexia, schizophrenia spectrum disorders, and neurological soft signs in children and adolescents. METHOD Month of birth and the diagnostic findings in question were examined based on charts from a clinic population of 585 boys. Odds ratios and etiological fractions were calculated. RESULTS Neurological soft signs showed a sporadic peak for June births and schizophrenia spectrum showed a peak for August and November. A smooth curve suggesting true seasonality was evident in dyslexia for births in May, June, and July. For different 5-year birth cohorts, early summer birth accounts for 24 to 71% of cases of dyslexia. CONCLUSIONS The authors suggest that viral infection, especially influenza, during the second trimester of pregnancy is the most attractive hypothesis to explain these findings. If this hypothesis is supported, immunization in women of child-bearing age could reduce the incidence of dyslexia. Secondary prevention could also be enhanced by early identification and treatment of children who were exposed in utero.