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Background: Incidence rates of thyroid cancer in children and young adults (age 0–19 years) have nearly doubled over a recent 15-year period in the United States. Children with thyroid cancer may require long-term therapy and surveillance and are at greater risk for second primary malignancies. High-dose exposure to ionizing radiation is the only known nongenetic risk factor; the vast majority of cases have an unknown etiology. Methods: We conducted a population-based nested case-control study to evaluate the relationship between a range of birth characteristics and the risk of pediatric thyroid cancer. Using linked birth records and cancer registry data from California, we included 1012 cases who were diagnosed with first primary thyroid cancer at the age of 0–19 years from 1988 to 2015 and 50,600 birth-year matched controls (1:50 case to control ratio). We estimated adjusted odds ratios (OR) and 95% confidence intervals (CI) by using multivariable logistic regression models applied to the full population and stratified by thyroid cancer subtypes (papillary and follicular), race/ethnicity (white and Hispanic), and age at diagnosis (0–14 and 15–19 years). Results: Hispanic ethnicity (OR: 1.20 [CI 1.01–1.42]), higher birth weight (OR: 1.11 [CI 1.04–1.18] per 500g), and higher maternal education (13–15 years OR: 1.35 [CI 1.09–1.68], 16+ years OR: 1.35 [CI 1.07–1.71]) were associated with an increased risk of pediatric thyroid cancer, while male sex (OR: 0.21 [CI 0.18–0.25]) and higher birth order (third or higher OR: 0.81 [CI 0.68–0.98]) were associated with a decreased risk. Some heterogeneity was observed across subtype, most notably an elevated OR with higher birth order for follicular thyroid cancer, in contrast to the reduced risk for this category among papillary thyroid cancer cases (p-value for interaction = 0.01). Hispanic ethnicity was a risk factor for papillary, but not follicular thyroid cancer (p-value for interaction = 0.07). Conclusions: In this population-based study of birth characteristics and pediatric thyroid cancer, we identified several important risk factors for pediatric thyroid cancer, including female sex, Hispanic ethnicity, higher birth weight, higher maternal educational attainment, and lower birth order. Our data provide new areas for replication and investigation of biological mechanisms for this poorly understood malignancy. [ABSTRACT FROM AUTHOR]