Heritability of subfertility among Danish twins.

<bold>Objective: </bold>To investigate the relative contribution of genetic and environmental components to subfertility.<bold>Design: </bold>Twin design using a quantitative genetic liability threshold model that splits the variation of subfertility into additive genetic effects, common environmental effects, and unique environmental effects.<bold>Setting: </bold>Not applicable.<bold>Patients: </bold>A total of 9053 Danish monozygotic and dizygotic same-sex twins aged 18+ years from nationwide twin surveys (twins born 1931-1976).<bold>Intervention: </bold>Not applicable.<bold>Main Outcome Measures: </bold>Time to pregnancy (TTP) restricted to first pregnancy as a binary outcome, with a cut-off point of 10 months.<bold>Results: </bold>Based on the Akaike information criterion, a model including additive genetic and unique environmental factors resulted in the best model fit. For females, the relative contribution of additive genetic factors to TTP was 28% (95% confidence interval [CI] 15%, 41%), whereas unique environmental factors explained 72% (95% CI 59%, 85%). For males, additive genetic factors explained 4% (95% CI 0%, 22%) of the variation in TTP, while unique environmental factors accounted for 96% (95% CI 78%, 100%). Results were overall similar for the crude model and consistent across surveys.<bold>Conclusion: </bold>Unique environmental factors explain most of the observed variation in subfertility, when measured as waiting time to pregnancy. [ABSTRACT FROM AUTHOR]