Recurrent pregnancy loss (RPL), defined as two to three spontaneous pregnancy terminations occurring before 12 weeks of gestation, affects approximately 1% of the general population. The causes may include congenital factors that originate with the quality of the gametes (sperm or oocyte) or the resulting embryo, or factors that originate within the uterus. Alterations of endometrial receptivity from endometriosis and/or endometritis, which are associated with impaired action of progesterone, have also been implicated in RPL. Finally, immunologic factors and thrombophilia, congenital and acquired, have also been suspected to cause RPL.