Cycle day, estrogen level, and lead follicle size: analysis of 27,790 in vitro fertilization cycles to determine optimal start criteria for gonadotropin-releasing hormone antagonist

Cycle day, E2 level, and follicle size at time of antagonist start are all independent predictors of a clinical pregnancy after IVF. Initiating antagonists when the E2 level is extremely low (<300 pg/mL) or extremely high (>1,100 pg/mL) significantly reduces the odds of pregnancy.