Prolutex in Frozen Embryo Transfer Cycles At the Blastocyst Stage (PROGEX)

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.