The incidence of severe early OHSS in patients was 16.3 % (7/43), 19.6 % (10/51), 18.0 % (7/39), 17.9 % (5/28), and 18.3 % (22/120) in the letrozole group, mifepristone group, GnRH-ant group, three-drug group, and control group, respectively. This evidence concerns the gene GNRH1 and ovarian hyperstimulation syndrome.