Increased Sperm Count Maintains High Population Frequency of Factor V Leiden.

Factor V Leiden (FVL) increases the risk of venous thrombosis and pregnancy loss in carriers. Nevertheless, this relatively old mutation is still prevalent in Caucasion populations, which could be explained by positive selection pressure. Men with FVL have previously been found to have higher fecundity (the time between marriage and first pregnancy). Whether this is caused by increased fertility is unknown. We therefore aimed to assess whether men with FVL have increased fertility.

We performed a pilot study in 19 men with FVL. For confirmation, we performed a cohort study among 961 consecutively included male partners of couples presenting at our university hospital for fertility workup between January 2000 and July 2007. We genotyped all participants and compared the outcome of the fertility workup between FVL carriers and non-carriers.

The pilot study showed increased sperm concentration and total sperm counts in the 19 FVL carriers as compared to men from the general population. In the cohort study, we identified 37 FVL carriers and 921 non-carriers. FVL carriers had higher total sperm counts and total motile sperm counts than non-carriers: 236 ×10⁶ (95% CI 158-292 ×10⁶) versus 163 ×10⁶ (95% CI 147-178 ×10⁶) and 81 ×10⁶ (95% CI 54-105 ×10⁶) versus 52 ×10⁶ (95% CI 48-57 ×10⁶), respectively.

Our results provide a possible explanation for the high prevalence of FVL among Caucasians. To our knowledge, this is the first study that indicates that an increased prevalence of a genotype is controlled by increased sperm counts.

No relevant conflicts of interest to declare.