The Swedish Multi-Generation Register and the Swedish Patient Register were used to follow all Swedish-born adoptees (born 1932 through 2004) that could be linked to both their biological and their adoptive parents (n = 80,214; 50% females). The risk of VVs was estimated in adoptees with at least 1 biological parent with VVs, but no adoptive parent with VVs (n = 187) compared with adoptees without a biological or adoptive parent with VVs (n = 1,758). The risk of VVs was also determined in adoptees with at least 1 adoptive parent, but no biological parent with VVs (n = 87), and in adoptees with both biological and adoptive parents affected (n = 21).
Adoptees from an affected biological parent, but no adoptive parent, were more likely to have VVs than adoptees from an unaffected biological or adoptive parent (standard incidence ratio [SIR] = 2.21; 95% CI, 1.91–2.55). The familial SIR for adoptees with both an affected biological parent and an adoptive parent was 4.58 (95% CI, 2.83–7.01). Adoptees with an affected adoptive parent but no biological parent were not at increased risk of VVs (SIR = 1.15; 95% CI, 0.92–1.42).
These novel findings suggest that genetic factors make a strong contribution to the familial transmission of VVs from parents to offspring, although familial environmental factors might contribute.