The influence of estradiol/follicle and estradiol/oocyte ratios on the outcome of controlled ovarian stimulation for in vitro fertilization

Objective. The aim of the study was to evaluate the influence of the ratios of estradiol (E2) to either the number of follicles >14 mm on the day of human chorionic gonadotropin administration (E2/follicle) or the number of oocytes retrieved (E2/oocytes) during controlled ovarian hyperstimulation (COH) with gonadotropin-releasing hormone (GnRH)-agonist (agonist group) and GnRH-antagonist (antagonist group), on the outcome of in vitro fertilization (IVF) cycles. Patients and methods. All consecutive women aged <35 years admitted to our IVF unit during a 6-year period with normal to high response to COH were retrospectively studied. Ovarian stimulation characteristics, number of oocytes retrieved, number of embryos transferred and pregnancy rate were assessed. Results. Six hundred and ninety consecutive IVF cycles were evaluated, 301 in the agonist group and 389 in the antagonist group. The ratios of E2/follicle and E2/oocyte were significantly higher in the agonist group (p < 0.001 for both). Moreover, while pregnancy rates within E2/oocyte ratio of 100-200 pg/ml were comparable between the agonist and antagonist groups, when E2/oocyte ratios were <100 pg/ml or >200 pg/ml, pregnancy rates were significantly higher in the agonist group. Furthermore, no difference in pregnancy rates was observed within the agonist group between different E2/oocytes ratios, while within the antagonist group, higher pregnancy rates were observed when comparing those with E2/oocyte ratio of 100-200 pg/ml with those with E2/oocyte ratio <100 pg/ml or >200 pg/ml. Conclusion. While E2/oocyte ratio cannot predict the success of GnRH-agonist protocol, patients undergoing GnRH-antagonist protocol should reach E2/oocyte ratio within the 100-200 pg/ml range in order to achieve the best IVF outcome.