Group A will include all the patient that will start with progesterone ((P), Utrogestan®) supplementation after 7 days of E2 (Progynova®) intake and that will perform the FET (after 13 days of E2 and 6 days of P).

Group B will include the patients that will perform 14 days of E2 intake (Progynova®, 6 mg per day (2 mg every 8 hours) those patients will be asked to perform a supplementary blood test and ultrasound on day 14 of E2 intake, afterwards, they will start with P supplementation (Utrogestan®, 800 mg per day, 400 mg every 12 hours) from day 15 of E2 for 6 days and they will get their FET day 20 of the cycle (after 14 days of E2 and 6 days of P)

All women included will take exogenous hormone with daily administration of Estradiol Valerate (Progynova®), the standard dose used will be 6 mg/day (Cobo et al., n.d.). On day 7 of endometrial preparation patients will be examined with vaginal ultrasound and blood test, to evaluate the endometrial thickness and the hormones levels. In both arms the FET will take place after 6 days of Progesterone supplementation (600 mg per day, vaginal administration of Utrogestan®) According to embryo quality after warming, embryo transfer to the uterine cavity will be performed on day 5 of development under ultrasound guidance whenever possible. Following embryo transfer, luteal support will be continued with vaginally administered P (Utrogestan®). All patients 12 days after the FET will undergo the pregnancy test (blood test to evaluate hCG).

The number of clinical pregnancies expressed per 100 initiated cycles, aspiration cycles or embryo transfer cycles. When clinical pregnancy rates are recorded, the denominator (initiated, aspirated or embryo transfer cycles) must be specified.

The number of deliveries that resulted in at least one live birth, expressed per 100 cycle attempts. In the case of assisted reproductive technologies (ART) interventions, they can be initiated cycles, insemination, aspiration cycles or embryo transfer cycles. When delivery rates are given, the denominator (initiated, inseminated, aspirated or embryo transfer cycles) must be specified.

The spontaneous loss of an intra-uterine pregnancy prior to 22 completed weeks of gestational age, over the number of transferred embryos.

the number of participants with positive hCG measured on a blood sample 12 days after ET

the number of patients with positive hCG but no embryo development assessed