The correlation of fecundability among twins: Evidence of a genetic effect on fertility?

Background. Numerous rare genetic conditions are known to influence fecundability in both males and females. It is less clear to what extent more subtle genetic differences influence fecundability on a population level. Methods. In 1994 a population-based survey was conducted among Danish twins born 1953-1982. Fecundability was assessed as the waiting time to pregnancy at the first attempt to achieve a pregnancy. Results. The reported time to pregnancy for males was slightly shorter than for females but there were no sex differences in intrapair similarity. We found an intrapair correlation in time to pregnancy for 645 monozygotic twin pairs (r = 0.22; 95% confidence interval = 0.12 to 0.32), but no intrapair correlation for 826 like-sex dizygotic twin pairs (r = 0.00; 95% confidence interval = -0.09 to 0.10) Conclusions. The correlation in time to pregnancy for monozygotic twins suggests genetic factors, although similarities in reporting behaviors could kilo be contributing to the correlation. The lack of correlation in time to pregnancy for dizygotic twins indicates that possible genetic factors of importance for fecundabitilty are acting nonadditively. Hence, it may prove difficult to identify specific gene variants that influence fecundability on a population level if their effects depend on gene-gene interactions.