In the HRT FET, estradiol valerate (Progynova) will be administered for endometrial preparation. If the endometrial thickness will be ≥ 6,5 mm during an ultrasound scan planned following an initial period of estrogen priming, micronized vaginal progesterone will be started (Utrogestan 800 mg/d). The transfer of a single blastocyst will be performed on the 6th day of progesterone administration. In a modified NC FET, there is no intake of medication but endocrine and ultrasound monitoring during the proliferative phase. Ovulation trigger with hCG will be considered when a dominant follicle between 16 and 20 mm will be observed and when the endometrial thickness will be ≥ 6,5 mm. The transfer of a single blastocyst will take place on the 7th day after hCG injection.~Serum progesterone levels will be evaluated at four different time points during the day of an artificially prepared frozen embryo transfer cycle (HRT FET cycle) and a modified natural cycle frozen embryo transfer cycle (mNC FET cycle) (at 08:00, 12:00, 16:00 and 20:00). Pairwise percentage differences in serum progesterone levels for each patient will be calculated. The intra-day variability of progesterone will be analyzed using mixed models. Pregnancy outcomes will also be assessed.
Serum progesterone levels will be evaluated at four different time points during the day of an artificially prepared frozen embryo transfer cycle (HRT FET cycle) and a modified natural cycle frozen embryo transfer cycle (mNC FET cycle) (at 08:00, 12:00, 16:00 and 20:00).The pairwise percentage differences in serum progesterone levels for each patient will be calculated. The intra-day variability of progesterone will be analyzed using mixed models. Pregnancy outcomes will also be assessed.