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.

Inclusion Criteria

All women with no history of infertility who agree to participate in the study:

1. Age: 18-35 years old, both inclusive
2. Regular menstrual cycles
3. In good health and not suffering from any mental or medical condition(s) that would preclude participation in the study.

Exclusion Criteria

Subjects who meet one or more of the following will not be considered eligible to participate in the pilot study:

1. Simultaneous participation in other clinical studies that, at the researcher's criteria, could interfere with the results of this study.
2. Taking oral contraceptives in the three months prior to signing informed consent.
3. Presence of uterine pathology (submucosal or intramural myomas >4 cm deforming cavity, endometrial polyps or müllerian anomalies) or adnexal pathology (communicating hydrosalpinx).
4. Background of thrombosis, breast cancer, systemic diseases.
5. Those unable to comprehend the investigational nature of the proposed study.

Patients will be evaluated by blinded gynecologist in order to prevent the application of personal criteria and protect the consistence of the study.