Natural Cycle vs. Modified Natural Cycle vs. Artificial Cycle Protocol for Endometrial Preparation.

The first ultrasound scan will be performed on the second to the fourth day of the menstrual cycle to identify any problem related to the patients' uterus or adnexa. The second ultrasound will be performed on the sixth day of the cycle. Daily ultrasound and serum estradiol and LH level evaluation will be performed when the mean diameter of the dominant follicle of ≥14 mm. LH surge initiation is defined as a concentration of 180% above the latest serum value available in that patient with a continued rise thereafter9 to a level of 20 IU/l or more10 detected by the ECLIA method (Roche Cobas® E 801, Roche Diagnostics, Germany). Embryo transfer will be scheduled by the time of the initiation of LH and embryo stages.

The first ultrasound scan will be performed on the second to the fourth day of the menstrual cycle to identify any problem related to patients' uterus or adnexa. A second ultrasound scan will be performed on the sixth day of the cycle; if there is at least one follicle with a diameter of ≥12 mm, an ultrasound scan will be performed daily. When the dominant follicle's mean diameter is ≥16 mm, human chorionic gonadotropin (Ovitrelle® 250 μg; Merck, Kenilworth, NJ, USA) will be injected to trigger ovulation. Embryo transfer will be scheduled by the time of the hCG injection and embryo stages.

The endometrium is prepared using oral estradiol valerate (Valiera®; Laboratories Recalcine) 8 mg/day, ranging from the second or fourth menstruation day. The endometrial thickness will be monitored from day six onwards, and vaginal progesterone (Utrogestan®; Besins) 800 mg/day will be initiated when endometrial thickness reaches ≥7 mm. Estradiol exposure must be lasting for ≥9 days before progesterone administration. Embryo transfer will be scheduled by the time of the initiation of progesterone and embryo stages.