Objectives: Assessment of the applicability of estimated serum inhibin A (INHA) and/or INHB versus estradiol (E2) levels to determine the triggering time during controlled ovarian stimulation (COS) that might allow the retrieval of the optimal number of oocytes.   Patients: 196 infertile women assigned to receive COS using the flexible antagonist protocol gave blood samples for ELISA estimation of serum levels of E2 and INHs on day-2 of the cycle and on the day of retrieval depending on having serum E2 levels >2000 pg/ml and the optimal number of follicles; 11-15 follicles of ≥16 mm in diameter.  The study outcome is the blinded distinguishing ability of estimated serum levels of E2 and INHs between cases that might have <11, 11-13, and 14-15 mature follicles on transvaginal ultrasonography (TUV) imaging on the day of triggering.    Results: Serum E2 levels at the time of triggering (>2000 pg/ml) showed sensitivity and specificity rates of 89.8% and 40.6% to distinguish women who had <11 mature follicles after COS. Serum levels of INHA (≥723 ng/ml) showed significantly higher diagnostic performance for differentiation between patients according to the number of mature follicles compared to serum E2 and INHB levels with significant area under the curve (AUC) for differentiating patients had <11 or >14 mature follicles, while E2 and INHB failed for this respect. Statistical analyses defined serum INHA as the significant predictor for canceling depending on the maturation of a low number of follicles after COS and for the presence of 14-15 mature follicles and deciding triggering and oocyte retrieval.    Conclusion:  Estimation of serum E2 is an unreliable marker for differentiating women according to several mature follicles. Estimated serum levels of INHA are the best biomarker for the identification of women who might have <11 follicles and women who might have ≥14 mature follicles with high specificity.