Patients will be stimulated according to the conventional flexible GnRH antagonist protocol for IVF. Alfa follitropin (150IU a day) will be started on the third day of the menstrual cycle. Treatment monitoring will be done with transvaginal ultrasound scans and serum determinations of estradiol and progesterone 5 days after the start of gonadotropins and every each day thereafter. Once the leading follicle reaches 13 mm in mean diameter 0,25mg of cetrorelix acetate will be administered daily. Once at least two follicles reach 18mm or more in mean diameter 250 micrograms of choriogonadotropin alfa will be administered and 36 hours latter patients will undergo follicle aspiration for IVF. Embryos will be cryopreserved (vitrification) on the third or fifth day of development. Two months after women will undergo uterine preparation for embryo transfer.

Patients will be monitored with daily transvaginal ultrasound scans from the tenth day of the menstrual cycle on. When the dominant follicle reaches a mean diameter of 16mm or more, patients will receive 250 micrograms of choriogonadotropin alfa subcutaneously. Daily transvaginal ultrasound scans will be done starting two days after the administration of the medication until a cohort of ovarian follicles between 4-6 mm is seen (follicular wave emergence). From this point on patients will undergo the same stimulation protocol as Controls, i.e., flexible GnRH antagonist protocol.

Patients will be monitored with daily transvaginal ultrasound scans from the tenth day of the menstrual cycle on. When the dominant follicle reaches a mean diameter of 16mm or more, patients will then undergo aspiration of the dominant and all follicles greater than 10mm in mean diameter. aspiration will be transvaginal ultrasound guided and under sedation, as for oocyte retrieval. Oocytes eventually obtained at this first aspiration will not be used for IVF. Daily transvaginal ultrasound scans will be done starting the day after the follicular aspiration until a cohort of follicles between 4-6 mm is seen (follicular wave emergence). From this point on patients will undergo the same stimulation protocol as Controls, i.e., flexible GnRH antagonist protocol.

Administration of 250 micrograms of choriogonadotropin alfa subcutaneously when the dominant follicle of a natural menstrual cycle reaches 16mm or more of mean diameter. Verify if this intervention is able to induce follicular wave emergence and synchronize the start of the flexible GnRH antagonist protocol of ovarian stimulation with the start of a follicular wave.

Evaluate if the aspiration of the dominant follicle is able to induce a follicular wave to emerge.

Evaluate if administration of hCG is able to induce ovulation of a dominant follicle larger than 16mm and to induce a follicular wave to emerge.

A follicular wave emergence is defined as an increase in the number of ovarian follicles smaller than 10mm seen at the transvaginal ultrasound scan after the interventions

Evaluate with periodic transvaginal ultrasound scan the size (mm), number and growth rate (mm/day) of ovarian follicles in each of the three groups

Evaluate blood levels of estradiol (pg/mL) and progesterone (ng/mL) during ovarian stimulation in each of the three groups at each visit to evaluate the progress of treatment.