Compared with the control group, the patients with moderate-to-critical OHSS in our center were characterized by a young age, a low BMI, ovulation disorders or PCOS, a low basal FSH level, a higher E2 level on the hCG trigger day, and follicles ≥ 12 mm on the trigger day of final oocyte maturation (Table 1). This evidence concerns the gene BRD2 and polycystic ovary syndrome.