Objective : To study the effect of changes in estradiol (E2 ) levels on the reproductive outcomes of patients with polycystic ovary syndrome (PCOS) undergoing controlled ovarian stimulation (COS) with gonadotropinreleasing hormone antagonist (GnRH⁃ant) protocol. Methods : A retrospective study was conducted involving 338 patients with polycystic ovary syndrome (PCOS) infertility who underwent GnRH⁃ant protocol for controlled ovarian hyperstimulation followed by in vitro fertilization ( IVF) or intracytoplasmic sperm injection ( ICSI) , and subsequently underwent their first frozen⁃thawed embryo transfer ( FET) . The clinical data of these patients were analyzed. Patients were grouped based on the changes in serum E2 levels on the first and fourth day of GnRH⁃ant administration (blood samples collected before GnRH⁃ant injection) : the E2 elevation group (Group A , E2 v~alue increased more than 30% , 165 cases) , the E2 stable group (Group B , E2 rate of change was within - 30% 30% , 162 cases) , and the E2 decline group (E2 value declined more than 30% , 11 cases , not included in statistical analysis due to small sample size) . The differences in demographic characteristics , ovulation induction outcomes , embryo outcomes and clinical pregnancy⁃related indicators were analyzed between Groups A and B. Results : There were no statistically significant differences in basic information such as age , duration of infertility , body mass index (BMI) , basal endocrine levels , and anti⁃Müllerian hormone (AMH) levels between groups A and B. Regarding embryo characteristics , there were no statistically significant differences in fertilization method and number of transferred embryos between groups A and B. However, group A had a higher total number of retrieved oocytes , normal fertilization rate , number of high⁃quality embryos , and rate of high⁃quality embryos compared to group B , with statistically significant differences ( P < 0. 05) . In terms of clinical pregnancy outcomes , there were no statistically significant differences in the incidence of severe ovarian hyperstimulation syndrome (OHSS) between groups A and B. However, group A had higher rates of clinical pregnancy , implantation , and live birth compared to group B , with statistically significant differences. Group A also had a lower rate of early miscarriage compared to group B , with statistically significant differences. Conclusion Choosing the GnRH⁃ant protocol for IVF/ICSI⁃FET in PCOS patients , if the blood E2 level increases by more than 30% after 4 days of adding the antagonist (blood sample collected before administering GnRH⁃ant) , the clinical pregnancy outcome will be better.