Abstract: Objective: To investigate the risk factors of ovarian hyperstimulation syndrome（OHSS） in those patients during the controlled ovarian hyperstimulation（ＣＯＨ） by a retrospective analysis, and compare further the early-onset OHSS and late-onset OHSS. Methods：Thirty six patients were hospitalized due to the moderate to severe OHSS from June 2002 to August 2010, including 25 early-onset OHSS patients and 11 late-onset OHSS patients. The high risk group included 76 patients who had the whole-embryo freezing or the one-embryo transfer due to their high risks of OHSS and did not suffer from OHSS. The control group included other 61 patients who had similar basic characteristics to the OHSS group，transferred more than 2 embryos and did not have OHSS. The level of serum estradiol was measured on the hCG injection day. The number of punctured follicle and the number of retrieved oocyte were counted. Results：① Patients with the early OHSS had higher E2 level on the hCG day than those patients with the late OHSS. ②The E2 levels on the hCG day and the numbers of oocyte in both the OHSS groups and the high risk group were higher than those in the control group. ③ There was significant difference in the number of retrieved oocyte among groups, while the OHSS group had significantly less retrieved oocyte than the high risk group. Conclusions：①The elevated level of E2 on the hCG day can be used as a auxiliary predictive factor for OHSS. ②The numbers of punctured follicle and harvested oocyte are sensitive factors for predicting OHSS. ③Multiple follicle development, especially many middle and small follicles, increases the risk of OHSS.

Key words: Ovarian hyperstimulation syndrome, Estradiol, Ovarian follicle, Risk factors