Thyroid cancer incidence is higher in women than men, especially during the reproductive years, for reasons that remain poorly understood. Using population-based registry data from four Nordic countries through 2015, we examined associations of perinatal characteristics with risk of maternal thyroid cancer. Cases were women diagnosed with thyroid cancer ≥2 years after last birth (n=7,425, 83% papillary). Cases were matched to controls (n=67,903) by mother's birth year, country, and county of

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... idence. Odds ratios (ORs) were estimated using conditional logistic regression models adjusted for parity. Older age at first pregnancy, postpartum hemorrhage (OR=1.18, 95% CI 1.08-1.29), and benign thyroid conditions (ORs ranging from 1.64 for hypothyroidism to 10.35 for thyroid neoplasms) were associated with increased thyroid cancer risk, as were higher offspring birth weight (per 1-kg increase, OR=1.17, 95% CI 1.12-1.22) and large-for-gestational-age (OR=1.26, 95% CI 1.11-1.43). Unmarried/non-cohabiting status (OR=0.91, 95% CI 0.84-0.98), maternal smoking (OR=0.75, 95% CI 0.67-0.84), and preterm birth (OR=0.90, 95% CI 0.83-0.98) were associated with reduced risk. Several factors (e.g., older age at first pregnancy, maternal smoking, goiter, benign neoplasms, postpartum hemorrhage, and hyperemesis gravidarum, neonatal jaundice) were associated with advanced thyroid cancer. These findings suggest that some perinatal exposures may influence maternal thyroid cancer risk.