Earlier studies have shown inherited thrombophilia to be a risk factor for recurrent implantation failure, raising the question of whether this risk is related to the underlying cause of unexplained infertility or to the mechanisms involved in the implantation process. When nine thrombophilic gene polymorphisms were compared among 92 women with the diagnosis of unexplained infertility and 60 fertile control women, women with a history of unexplained infertility displayed a higher prevalence of MTHFR C677T polymorphisms than control women.