INTRODUCTION
PCOS is a common endocrine disorder that can affect endometrial receptivity through a variety of ways, consequently leading to low pregnancy rate and high abortion rate in patients. Importantly, endometrial receptivity and embryo quality are two main factors affecting the success rate of FET in PCOS patients.
Synchronization between receptive endometrium and embryo development is critical for pregnancy establishment. Natural protocol is not commonly used since women with PCOS usually have irregular menstruation or oligo-anovulation, but artificial cycle and ovarian stimulation have been wildly applied. Hormone replacement therapy (HRT) is used in women with PCOS in an artificial cycle where the endometrium is prepared with programmed estrogen and progesterone, leading to a fixed time for ET. Stimulation protocols use ovulation stimulation medications, including clomiphene citrate (CC), letrozole, and gonadotropins to mimic the natural process of follicular development and facilitate endogenous estradiol through ovulation induction. HCG is commonly employed as a surrogate LH surge to stimulate the maturation of oocyte and increase the endometrial receptivity.