A high proportion of immature oocytes in a cycle cohort does not compromise embryo development or live birth rates after ICSI

Does a high proportion of immature oocytes impact embryo development and live birth rates in IVF-ICSI cycles? While a high proportion of immature oocytes is associated with lower blastocyst formation and reduced preimplantation genetic testing for aneuploidy (PGT-A) utilization, live birth rates remain comparable when key confounders-such as age, BMI, gonadotropin dosage, and metaphase-II (MII) count-are balanced, but cycles with a very low MII proportion resulted in fewer embryo transfers, which is quantitatively limiting, even if embryo quality appears unaffected. Previous studies have linked a lower proportion of mature oocytes (MII) to decreased fertilization rates, abnormal embryo development, and lower pregnancy and live birth rates. However, it remains unclear whether these outcomes are due to quantitative limitations (fewer mature oocytes available) or qualitative deficiencies (intrinsic oocyte quality issues). The impact of high proportions of immature oocytes on downstream IVF outcomes remains controversial. Retrospective cohort study at a single academically affiliated fertility clinic between December 2014 and December 2023. 22 117 patients undergoing IVF-ICSI cycles were categorized into three groups based on the proportion of mature (MII) oocytes: very low MII proportion (≤0.25), low MII proportion (≤0.5), and 100% MII oocytes. Propensity score matching (PSM) was applied in two phases: first, adjusting for age, BMI, gonadotropin dosage, and anti-Müllerian hormone levels; second, adjusting for age, BMI, gonadotropin dosage, and MII count. Cycles with a very low MII proportion (≤0.25) had significantly lower blastocyst formation-mean 1.32 (SD 2.77; 95% CI: 1.03-1.61) versus 3.10 (SD 3.22; 95% CI: 2.77-3.43; P < 0.001)-and reduced PGT-A utilization-mean percent PGT-A normal 34% (95% CI: 30-38%) versus 40% (95% CI: 36-44%; P < 0.001). However, after PSM, live birth rates per an embryo transfer did not differ significantly between very low MII and perfect MII groups for both cryopreserved transfers (41.6% versus 30%, P = 0.68) and fresh embryo transfers (46.1% versus 38%, P = 0.69). The low MII proportion group (≤0.5) showed a trend toward lower cryopreserved transfer rates (44% versus 53%, P = 0.05), while fresh transfer rates remained comparable (30% versus 26.6%, P = 0.55). This study is limited by its retrospective nature and reliance on electronic medical records. While PSM effectively reduced confounding, inherent oocyte quality markers (e.g. mitochondrial function) were not directly measured. Additionally, the non-uniform distribution of MII proportions across groups may have influenced statistical power. These results underscore the importance of personalized ovarian stimulation protocols to optimize mature oocyte (MII) yield and support successful outcomes, even in patients with high proportions of immature oocytes. The study was not funded. No competing interests. N/A.