Increased sperm count may account for high population frequency of factor V Leiden

Background: Factor V Leiden (FVL) increases the risk of venous thrombosis and pregnancy loss in carriers. Nevertheless, this relatively old mutation is 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 sperm counts in men with FVL is unknown. Objectives: To assess whether men with factor V Leiden have increased sperm counts. Patients/methods: We performed a prospective cohort study among 1139 consecutively included male partners of subfertile couples presenting at our university hospital for fertility workup between January 2000 and July 2007. All potential candidates who gave informed consent were included, irrespective of their fertility workup. In this retrospective analysis, we excluded participants with known causes of spermatogenic function or azoospermia. Subsequently, we genotyped all participants and compared sperm counts between FVL carriers and non-carriers. Results: 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 x 106 (95% CI 158-292 x 106) vs. 163 x 106 (95% CI 147-178 x 106) and 81 x 106 (95% CI 54-105 x 106) vs. 52 x 106 (95% CI 48-57 x 106), respectively. Conclusions: To our knowledge, this is the first study that indicates that an increased incidence of a genotype may be controlled by increased sperm counts. However, the finding that men with FVL had higher total (motile) sperm counts was not statistically significant and needs confirmation in other studies