Background: The POSEIDON criteria classify IVF/ICSI patients with hyporesponse, but their clinical value and stability remain uncertain. Objectives: This study evaluates the POSEIDON criteria using Anti-Müllerian Hormone (AMH) as an ovarian reserve biomarker. It compares the Cumulative Live Birth Rate (CLBR) per IVF/ICSI cycle in POSEIDON-defined hyporesponders versus normal responders, assesses whether redefining suboptimal response as 4–7 oocytes improves classification, and examines how many patients in POSEIDON groups transition to normal responders after adjusted stimulation protocols. Methods: A retrospective cohort study of 800 women aged 19-43 undergoing their first IVF/ICSI cycle at Antwerp University Hospital (2018-2022). Participants were post-hoc categorized into POSEIDON groups (n= 532, 66.5%) or a control group (n=268, 33.5%) based on AMH, age, and retrieved oocytes. Results: CLBR in POSEIDON groups was significantly lower than in normal responders (24.1% vs. 56.8%; p<0.001). Younger groups 1 (36.5%) and 3 (24.4%) had higher CLBR than groups 2 (22.4%) and 4 (11.3%). Adjusted criteria yielded similar patterns. After modified stimulation, 39% of POSEIDON patients transitioned to normal responders, while 38% of initial normal responders shifted to POSEIDON. Gonadotropin dose and protocol type influenced reclassification. Conclusions: Patients in POSEIDON groups have a significantly lower prognosis following IVF/ICSI compared to normal responders, with CLBR variation driven by age and oocyte number. The results confirm the importance of oocyte quality over quantity. The instability of POSEIDON classifications calls for reconsideration of their clinical utility, based on the observation that patients transition in and out of the POSEIDON classification.