Heritability of subfertility among Danish twins.

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, common environmental effects, and unique environmental effects.

SETTING

Not applicable.

PATIENTS: A total of 9053 Danish monozygotic and dizygotic same-sex twins aged 18+ years from nationwide twin surveys (twins born 1931-1976).

INTERVENTION: Not applicable.

MAIN OUTCOME MEASURES: Time to pregnancy (TTP) restricted to first pregnancy as a binary outcome, with a cut-off point of 10 months.

RESULTS

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.

CONCLUSION

Unique environmental factors explain most of the observed variation in subfertility, when measured as waiting time to pregnancy.