Stage one of treatment protocol First stimulation performs by Clomiphene citrate (Ovumid®, Iran Hormones Company) 25 mg/day with co-treatment of Letrozole (Letrofem®, Iran Hormones Company) 2.5 mg/day are given from cycle day 3 onwards. Letrozole is only given for 4 days and clomiphene citrate is continuously used before the trigger day. Patients start to inject human menopausal gonadotrophin (HMG) (Menopur®, Ferring, Switzerland) 150 IU every other day beginning on cycle day 6. When one or two dominant follicles (18 mm in diameter) observed, the final stage of oocyte maturation will be induced with triptorelin 100 μg (Decapeptyl®; Ferring GmbH, Germany) follows by ibuprofen 600 mg (Ibuprofen-Najo® , Coated Tablets, Najo Company, Iran) is used on the day of oocyte maturation triggering and the day after. After the first oocyte retrieval, human menopausal gonadotrophin and letrozole are administrated to stimulate follicle development, and oocyte retrieval will be carried out a second time when dominant follicles have matured. All highest-quality embryos (including grade 1 and grade 2, eight-cell blastomere embryos) will be cryopreserved by vitrification method on the third day after oocyte retrieval.