OBJECTIVE: To investigate the relative contribution of genetic and environmental components to subfertility. DESIGN: Twin design using a quantitative genetic liability threshold model that splits the variation of subfertility into additive genetic effects (A), common environmental effects (C) and unique environmental effects (E). SETTING: Two nation-wide twin cohorts. PATIENT(S): A total of 9,053 Danish monozygotic (MZ) and dizygotic (DZ) same-sex twins aged 18+ from nationwide twin surveys (twins born 1931-1976). INTERVENTION(S): Not applicable. MAIN OUTCOME MEASURE(S): Time to pregnancy (TTP) restricted to first pregnancy as a binary outcome, with a cut-off point of 10 months. RESULT(S): Based on the Akaike Information Criterion, the AE 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% CI 15-41%), while 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. CONCLUSION: Unique environmental factors explain most of the observed variation in subfertility, when measured as waiting time to pregnancy.