AbstractThe association between intrauterine growth restriction and cardiovascular disease (CVD) later in life might be confounded by familial factors. We conducted a binational register-based cohort study to assess associations of birth weight for gestational age (GA), a proxy for intrauterine growth restriction, and GA with CVD risk in early adulthood, before and after addressing familial factors via sibling comparison. We included 3,410,334 live nonmalformed singleton births from Sweden (1973–1996) and Denmark (1978–1998). During a median follow-up period of 10 years from age 18 years onwards, 29,742 individuals developed incident CVD (hypertension, ischemic heart disease, or cerebrovascular disease). Compared with individuals born with appropriate birth weight for GA (AGA; 10th–90th percentiles) or full term (39–40 gestational weeks), individuals born severely small for GA (SGA; ≤3rd percentile) or preterm (22–36 weeks) were at increased risk of CVD (hazard ratio (HR) = 1.38 (95% confidence interval (CI): 1.32, 1.45) and HR = 1.31 (95% CI: 1.25, 1.38), respectively). The association was attenuated when comparing individuals born SGA with their AGA siblings (HR = 1.11, 95% CI: 0.99, 1.25) but remained robust when comparing individuals born preterm with their term siblings (HR = 1.21, 95% CI: 1.07, 1.37). Our findings suggest that both SGA and preterm birth are associated with CVD risk in early adulthood, with greater familial confounding noted for SGA birth.