Month of Conception and Learning Disabilities: A Record-Linkage Study of 801,592 Children.
Learning disabilities have profound, long-lasting health sequelae. Affected children born over the course of 1 year in the United States of America generated an estimated lifetime cost of $51.2 billion. Results from some studies have suggested that autistic spectrum disorder may vary by season of birth, but there have been few studies in which investigators examined whether this is also true of other causes of learning disabilities. We undertook Scotland-wide record linkage of education (annual pupil census) and maternity (Scottish Morbidity Record 02) databases for 801,592 singleton children attending Scottish schools in 2006-2011. We modeled monthly rates using principal sine and cosine transformations of the month number and demonstrated cyclicity in the percentage of children with special educational needs. Rates were highest among children conceived in the first quarter of the year (January-March) and lowest among those conceived in the third (July-September) (8.9% vs 7.6%; P < 0.001). Seasonal variations were specific to autistic spectrum disorder, intellectual disabilities, and learning difficulties (e.g., dyslexia) and were absent for sensory or motor/physical impairments and mental, physical, or communication problems. Seasonality accounted for 11.4% (95% confidence interval: 9.0, 13.7) of all cases. Some biologically plausible causes of this variation, such as infection and maternal vitamin D levels, are potentially amendable to intervention.