Transvaginal ultrasound evaluation performs after oocyte retrieval to determine whether to continue the second ovarian stimulation. The criterion for continued stimulation is the presence of at least two antral follicles 2-8 mm in diameter. A total of 225 IU HMG (Menopur®, Ferring, Switzerland) and letrozole 2.5 mg (Letrofem®, Iran Hormones Company) is administered daily from the day of, or the day after, oocyte retrieval. The initial second stage follicular monitoring is conducted 5-7 days later, and then for follow up every 2-4 days, by using a transvaginal ultrasound evaluation, to record the number of developing follicles. Letrozole administration is discontinued when the dominant follicles reached diameters of 12 mm, given that large follicles have redundant LH and FSH receptors, and good response to exogenous hormone stimulations. Daily administration of medroxyprogesterone acetate 10 mg (Progestrone®, 5mg bid; Aboureihan, Iran) is added beginning on stimulation day 12 for cases in which post-ovulation follicle size is smaller than 14 mm in diameter and stimulation needed to continue for several more days. This performs to postpone menstruation and avoid oocyte retrieval during menstruation, to prevent the risk of infection from the procedure. When three dominant follicles reached diameters of 18 mm or one mature dominant follicle exceeded 20 mm, the final stage of oocyte maturation is induced again with triptorelin 100 μg (Decapeptyl®; Ferring GmbH, Germany) by injection. Again, ibuprofen 600 mg (Ibuprofen-Najo® , Coated Tablets, Najo Company, Iran) is used on the day of oocyte maturation triggering and the day after. Transvaginal ultrasound-guided oocyte retrieval was conducted 32-36 h after GnRH agonist administration. All retrieved oocytes were treated same as in the study stage one.