Serum Progesterone on the Day of Embryo Transfer and Pregnancy Rate.

Inclusion Criteria 1. Age \<50 years old 2. Hormonal Replacement therapy cycle for embryo transfer in any of the following situations: 1. Embryos in fresh cycle with donated oocytes. 2. Subsequent embryo transfers of vitrified embryos in cycles of donated oocytes. 3. Frozen embryo transfers in cycles with autologous oocytes. 3. Endometrial line: \>6.5-7mm (with trilaminar aspect) in proliferative phase, before introducing progesterone. 4. Administration of natural micronized progesterone intravaginally (400 mg/12 hours for 5 complete days (10 dosis), before the embryo transfer and maintained until pregnancy week 12. 5. Embryo transfer of blastocyst (day 5-6). 6. Signed Informed Consent Exclusion Criteria 1. Uterine pathology (submucous o intramural fibroids (\>4 cm) or which deform the uterine cavity; endometrial polyps or Müllerian anomalies; or adnexal anomalies (communicating hydrosalpinx). 2. Simultaneous participation in another clinical study. 3. Participation in another clinical study before the inclusion in the present study, and which could affect the objectives of the present study.