Genetic epidemiology of twinning: a population-based study.

We report on kinship analysis of mothers with twins and a study of their ancestors and inbreeding. A pair-matched case-control design supports the analyses. Nearly as high a number of ancestors was found at the origin of cases as in control individuals. Study cases were less inbred than control subjects, but their kinship was higher, especially for unlike-sex twins born after age 30 years of mothers and, but less so, for like-sex twins born to mothers younger than 30 years. The results indicate a genetic origin of both types of twins and possibly a highly spread gene with dominant or co-dominant expression.