Endometrial preparation for embryo transfer (ET) will be started after oocyte retrieval by giving 800 mg/day natural progesterone vaginal supplement + 4 mg/day estradiol oral supplement. The patients will be randomly divided into 2 groups for luteal phase support: (i) Group A (GnRHa group) in which the luteal phase support will be continued by the same regimen started on the day of oocytes retrieval until 2 weeks after embryo transfer (ET) with subcutaneous administration of a single dose (0.2 mg) of GnRHa (Triptorelin) 6 days after oocyte retrieval, and (ii) Group B (control group) in which the luteal phase support will be continued only by the same regimen started on the day of oocytes retrieval until 2 weeks after ET