Background <p>Inhibin B (InhB) is secreted by granulosa cells and reflects early follicular activity. While anti-Müllerian hormone (AMH) and antral follicle count (AFC) are established ovarian reserve markers, the role of bioavailable inhibin B (bio-InhB) in predicting oocyte yield remains uncertain. This is particularly relevant in women with low ovarian reserve, where reliable prediction of ovarian response is clinically important.</p> Objective <p>To assess whether baseline bio-InhB levels correlate with oocyte yield and add predictive value beyond AMH and AFC in women with low ovarian reserve undergoing IVF/ICSI.</p> Methods <p>This was a prospective observational study including 72 women with AMH &lt; 1.1&#xa0;ng/mL undergoing ovarian stimulation for IVF/ICSI using a GnRH-antagonist protocol. Baseline serum bio-InhB was measured on cycle day 2/3. Ovarian response was assessed by the number of cumulus-oocyte complexes (COCs) and metaphase II (MII) oocytes retrieved.</p> Results <p>Bio-InhB showed moderate correlation with AFC (rs = 0.38), AMH (rs = 0.35), and oocyte yield (COCs rs = 0.32; MII rs = 0.35). Women with high bio-InhB levels had significantly higher AFC and COC counts. However, in multivariate analysis, bio-InhB was not a significant predictor of oocyte yield, and its inclusion in predictive models did not improve accuracy based on Root Mean Square Error (RMSE) comparison (<i>p</i> = 0.272).</p> Conclusion <p>Although bio-InhB correlates with ovarian reserve markers and oocyte yield, it does not improve predictive accuracy beyond AMH and AFC. It may still serve as a complementary marker in selected cases with discordant ovarian reserve profiles or unreliable AMH results, such as in cases of AMH mutations.</p> Trial registration <p>Clinicaltrials.gov (NCT03826888)</p>