Background and Aim: Cerebral palsy (CP) is the most prevalent, severe, costly neuro-motor disability in children, and it is permanent. The majority of CP cases have an unknown etiology. This is the first study to investigate whether season of conception is associated with CP in California. Methods: We analyzed a California birth cohort of 5,089,216 children born in 2005-2015. A total of 5,508 children diagnosed with CP were identified by linking their California birth records with the diagnostic records from the California Department of Developmental Services. We used multivariate logistic regression analyses to estimate the odds ratio (OR) and 95% confidence interval (CI) for CP according to the month and season of conception, controlling for maternal characteristics (age, race/ethnicity, education), child’s sex, and the year of conception. Month of conception was estimated from the child’s date of birth and length of gestation and was grouped into seasons (winter: Jan-March, spring: April-June, summer: July-Sept, fall: Oct-Dec). We performed stratified analyses by child’s sex (females, males), disease subtypes (spastic, ataxic, hypotonic, other), limb involvement (unilateral, bilateral), and geographical locations of birth (Northern, Central, Southern) in California. Results: Having been conceived in winter and spring was associated with 9% higher odds of having CP (OR=1.09, 95% CI winter: 1.01-1.18, spring: 1.01-1.17) compared to summer conceptions. When considering months, consistent increases of CP with >10% higher odds were observed for each month from January to June compared to September, except for March. We observed no apparent differences in the stratified analyses. Conclusions: This is the first study to show winter and spring conceptions to be associated with higher CP occurrence in the offspring. Environmental factors and/or infections that predominate in winter/spring in California may underly this increased CP risk and need to be further explored. Keywords: Cerebral Palsy, Seasonal Variation, Environment, Neurodevelopment, Birth cohort
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