Demographic and birth characteristics

We abstracted numerous variables from birth records. Factors were selected because of reported associations with pediatric thyroid cancer, other childhood cancers, adult thyroid cancer, or a plausible influence on thyroid hormone levels (22–26). Variables pertaining to the characteristics of the child included: year of birth in categories (1978–1982, 1983–1987, 1988–1992, 1993–2011), sex (male, female), and race (white, Hispanic, non-White and non-Hispanic), birth weight in grams (modeled continuously per 500 g), and gestational age in weeks (22–36, 37–41, 42–44, unknown). Parental characteristics included maternal age in years (<20, 20–24, 25–29, 30–34, 35+ years), maternal education in years (≤8, 9–11, 12, 13–15, 16+, unknown years), mother's place of birth (United States, foreign), and paternal age in years (<25, 25–29, 30–34, 35–39, 40+ years). Characteristics pertaining to the birth process included: birth plurality (singleton, multiple), birth order (first, second, or third and above), mode of delivery (vaginal, Cesarean section), presence of a congenital abnormality (no, yes), history of miscarriage/stillbirth (never, ever, unknown), history of maternal complications during pregnancy (never, ever, unknown), and history of Cesarean section (never, ever, unknown).