Hence, we decided to evaluate the effect of the aromatase inhibitor, letrozole, versus gonadotrophin-releasinghormone (GnRH)-antagonist (ganirelix acetate) on prevention of severity of OHSS and reduction in serum estradiol(E2) levels when administered during the luteal phase after oocyte retrieval in IVF/ICSI cycles. This evidence concerns the gene CYP19A1 and ovarian hyperstimulation syndrome.