Gestational age and risk of venous thromboembolism from birth through young adulthood.

Background: Preterm birth has been associated with increased risk of venous thromboembolism (VTE) in infancy, but the longer-term risk is unknown. Our aim was to examine this association from birth through young adulthood.

Methods: National cohort study of 3,571,574 individuals who were live-born in Sweden from 1973 through 2008, including 206,844 born preterm (gestational age <37 weeks), and followed up to 2010 (ages 0-38 years). The main outcome was VTE.

Results: A total of 7519 (0.2%) individuals were diagnosed with VTE in 70.8 million person-years of follow-up. Low gestational age at birth was associated with VTE in infancy (ages <1 year: adjusted hazard ratio 47.16 [95% confidence interval 21.30-104.42] for 22 to 27 weeks, 5.54 [2.53-12.12] for 28 to 33 weeks, 3.54 [2.07-6.06] for 34 to 36 weeks, 1.00 for 37 to 41 weeks [reference]), early childhood (ages 1-5 years), and young adulthood (ages 18-38 years: adjusted hazard ratio 2.76 [1.43-5.31] for 22 to 27 weeks, 1.53 [1.24-1.89] for 28 to 33 weeks, 1.24 [1.10-1.40] for 34 to 36 weeks, and 1.00 for 37 to 41 weeks [reference]), but not in late childhood (ages 6-12 years). Very preterm (<34 weeks) but not late preterm birth (34-36 weeks) was also associated with VTE in adolescence (ages 13-17 years). After further adjustment for comorbidities, these associations were attenuated, but most remained significantly elevated.

Conclusions: In this large national cohort, low gestational age at birth was associated with increased risk of VTE in infancy, early childhood, and young adulthood.