Objectives To identify infant, obstetrical and maternal characteristics associated with arterial ischaemic stroke (AIS) and venous thromboembolism (VTE) in infancy (<1 year).

Design Nationwide, population-based nested case-control study. All infants with a verified first-time diagnosis of AIS, VTE or both in Denmark through the years 1994–2006 were included, and 10 population controls were selected for each case.

Results Case-infants presented with AIS (n=71) or VTE (n=38). AIS in infancy was associated with primiparity (adjusted OR 5.9 CI 95% 3.0 to 11.6)), delivery by an emergency caesarean section (adjusted OR 1.9 (CI 95% 1.0 to 3.3)), and post-term birth (adjusted OR 2.2 (CI 95% 1.1 to 4.8)). Male sex was associated with an increased risk of AIS among neonates (crude OR 1.8 (CI 95% 1.0 to 3.4)) but not among later born (crude OR 0.6 (CI 95% 0.2 to 1.4)). Risk factors for VTE in infancy included preterm birth (adjusted OR 5.5 (CI 95% 1.8 to 16.9)), low Apgar score (adjusted OR 9.2 (CI 95% 1.9 to 45.2)), and multiple births (adjusted OR 7.1 (CI 95% 1.1 to 48.1)). Previous maternal thromboembolism and pregnancy-related disorders were not associated with the risk of thromboembolism in the children.

Conclusion Several apparently independent infant, obstetrical and maternal characteristics were associated with thromboembolism in early life.