The objective of this proof of concept clinical trial is to evaluate the impact of two different progesterone treatments for endometrial preparation (25 mg/twice-a-day, subcutaneous injection, and 200 mg/three times a day, vaginal administration) on the clinical pregnancy rate in women undergoing frozen embryo transfer (FET) at blastocyst stage.

Inclusion Criteria:

• Women attending the clinics to undergo a frozen blastocyst embryo transfer, having given written informed consent, with the following characteristics:
• 18-49 years of age for subjects undergoing ET with donated oocytes (both inclusive);
• 18-37 years of age for subjects undergoing ET with autologous oocytes (both inclusive);
• BMI <32 kg/m2;
• Adequate endometrium preparation (Endometrial thickness > 7 mm) and E2 levels ( >100 pg/ml) on the day progesterone treatment is started;
• P4 levels <1.5 ng/ml on the day progesterone treatment is started;
• Transfer of 1 or 2 frozen embryos at blastocyst stage
• Transfer of frozen embryos of quality A and/or B according to Gardner criteria1;
• Semen from ejaculation either from the partner or from a bank
• ≤ 3 previous ET (frozen and fresh) with no pregnancy
• Normal uterine cavity (i.e. no polyp or protruding sub-mucosal fibroid).

Exclusion Criteria:

• Presence of functional follicles > 10 mm of diameter on the day progesterone treatment is started;
• Intramural uterine fibroids that distort the uterine cavity or polyps >1 cm;
• Stage III or IV endometriosis (endometriomas);
• Hydrosalpinx;
• Pregnancy or lactation
• Malformations of the sexual organs incompatible with pregnancy;
• Patients affected by pathologies associated with any contraindication of being pregnant;
• Known allergy to progesterone preparations or their excipients;
• Uncontrolled adrenal or thyroid dysfunction;
• Undiagnosed vaginal haemorrhage;
• History of, or current arterial disease;
• Patients with hepatic impairment;
• HIV, Hepatitis B Virus or Hepatitis C Virus seropositive;
• Neoplasias (current) or history of neoplasia that may be responsive to progesterone;
• High grade cervical dysplasia;
• Active thrombophlebitis or thromboembolic disorders, or a history of hormone-associated thrombophlebitis or thromboembolic disorders;
• Currently dependent on alcohol, drugs or psychotropic drugs
• History of recurrent pregnancy loss defined as 3 or more spontaneous miscarriages wherein pregnancy developed to a minimum of a gestational sac on transvaginal ultrasonography;
• Participation in a concurrent clinical trial or another trial within the past 2 months;
• Use of concomitant medications that might interfere with the study evaluation: hormonal treatments other than those used in the study, except thyroid hormones.