The purpose of this study is to compare the natural cycle (without any medication) with the well-established artificial cycle in an egg donation program.

Oocyte donation is an assisted reproduction technique well established. In patients with ovarian function, it is necessary to synchronize the cycle of the egg donor with the recipient, usually through endometrial preparation of the recipient by artificial cycle, by administering a GnRH agonist on day 21 of cycle and then administered increasing doses of estrogen therapy to achieve adequate endometrial thickness. The necessity of synchronization between donors and recipient, has made possible not routinely the natural cycle for oocyte donation. The investigators have recently introduced oocyte vitrification and it allows us to plan the egg donation in a different way. Now the investigators can previously cryopreserved donor oocytes and at the time that the investigators have a compatible receiver, then, plan the donation In this study it will be possible compare the results of oocyte donation cycles in terms of pregnancy rate, implantation and liveborn, depending on whether the recipient has made an artificial cycle of preparation endometrial or a natural cycle.