The aim of this randomized controlled trial is to compare the efficacy of extended high dose letrozole regimen (5 mg /day for the first 5 days of cycle and 2.5 mg/day for the subsequent 3 days ) with short low dose letrozole regimen (2,5 mg/day from cycle day 3 to 7 ) as an adjuvant to GnRH antagonist protocol in the management of patients with poor ovarian response undergoing IVF-ET.