Venous thromboembolism (VTE) clusters in families, but the familial risk of VTE has not been determined among adoptees. The aim was to disentangle the contributions of genetic and environmental factors to the familial transmission of VTE.Methods and Results—
The Swedish Multi-Generation Register was used to follow all Swedish-born adoptees born from 1932 to 2004 (n=80,214) between January 1, 1964, and December 31, 2010, for VTE. The risk of VTE was estimated in adoptees with ≥1 biological parent with VTE compared with adoptees without a biological parent with VTE. The risk of VTE was also estimated in adoptees with ≥1 adoptive parent with VTE compared with adoptees without an adoptive parent with VTE. Adoptees with ≥1 biological parent with VTE (n=137) were more likely to have VTE than adoptees without a biological parent with VTE (standardized incidence ratio) 1.51 (95% confidence interval, 1.27–1.79). The standardized incidence ratio for VTE was highest for adoptees with a biological parent diagnosed with VTE before the age of 50 years (standardized incidence ratio=2.03, 1.24–3.14). In contrast, adoptees with ≥1 adoptive parent with VTE (n=156) were not at increased risk of VTE (standardized incidence ratio=1.07, 0.91–1.25).Conclusions—
These novel findings suggest that genetic factors make a stronger contribution to the familial transmission of VTE from parents to offspring than family environmental factors.