After so many years conducting artificial endometrial preparation cycles for embryo transfer, there is no clear indication about which is the optimal dose of exogenous progesterone in this scenario to optimize the outcome. Taking into account that the luteal phase can be controlled by measuring serum P levels (not done until now), the next step is to find out which is the best dose and route of administration of exogenous progesterone for luteal phase in artificial cycles. Therefore, the aim of this experimental study is to compare the endometrial function and structure, as well as the serum P levels according to the use of different types of exogenous progesterone available on the market depending on their doses and route of administration (vaginal, subcutaneous or intramuscular). The endometrial receptivity status will be compared in the different artificial cycles with the one observed in a natural cycle, without exogenous progesterone (only the endogenous one) as a control group. Endometrial receptivity will be analysed by means of endometrial function and structure, but not by pregnancy outcome as in this study an embryo cannot be replaced in the uterus because an endometrial biopsy needs to be done to do this type of research.