Background Many cardiovascular conditions exhibit seasonality in occurrence and mortality, but little is known about the seasonality of venous thromboembolism. Methods Using Danish registries, we identified all patients with deep vein thrombosis, pulmonary embolism, splanchnic vein thrombosis, cerebral vein thrombosis, and retinal vein thrombosis during 1977-2016. We tallied monthly deaths occurring within 90 days of the venous thromboembolism diagnosis. We estimated peak-to-trough ratios and timing of the peak of both diagnoses and deaths summed over all years of the study period. The departure from 1.0 of the peak-to-trough ratio measures the intensity of any seasonal pattern. Results We estimated a peak-to-trough ratio of 1.09 (95% confidence interval: 1.07-1.11) for deep vein thrombosis and 1.22 (1.19-1.24) for pulmonary embolism occurrence. The peak-to-trough ratios for splanchnic vein thrombosis, cerebral vein thrombosis, and retinal vein thrombosis occurrence were 1.10 (1.01-1.20), 1.19 (1.00-1.40), and 1.12 (1.07-1.17), respectively. The occurrence of all conditions peaked during winter or fall. In time trend analyses, the peak-to-trough ratio increased considerably for splanchnic vein thrombosis, cerebral vein thrombosis, and retinal vein thrombosis occurrence. In associated mortality, the peak-to-trough ratio for deep vein thrombosis was larger (1.15, 1.07-1.23) than that for pulmonary embolism (1.04, 1.01-1.08). Discussion Excess winter risks were modest, but more marked for pulmonary embolism occurrence than for deep vein thrombosis occurrence. The seasonal pattern intensified throughout the study period for splanchnic vein thrombosis, cerebral vein thrombosis, and retinal vein thrombosis. The winter peak in mortality following pulmonary embolism was smaller than that for deep vein thrombosis.
