L26/P-168 Blastocyst developmental characteristics following in vitro maturation of MI oocytes versus in vivo–matured MII oocytes: a retrospective multicenter study

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Abstract Study question Do blastocysts derived from MI oocytes matured in vitro differ in developmental stage and morphological quality from those derived from in vivo matured MII oocytes? Summary answer Blastocysts from MI oocytes matured in vitro show similar blastulation timing but earlier expansion stages and poorer morphology, mainly affecting the trophectoderm. What is known already Immature oocytes retrieved at oocyte pick-up may complete nuclear maturation in vitro and reach metaphase II prior to ICSI; however, concerns persist regarding their developmental competence. Previous studies evaluating these oocytes have reported inconsistent findings and are often limited by small sample sizes or inadequate control of confounding factors. The effect of in vitro maturation of MI oocytes on blastocyst expansion dynamics and cellular quality remains insufficiently characterized in large, well-controlled studies. Study design, size, duration Retrospective multicenter propensity score–matched blastocyst-level cohort study including 30,566 blastocysts obtained from IVF treatment at two clinics between 2018 and 2023 were included. After 1:1 matching on oocyte age, oocyte type, oocyte origin, day-3 assisted hatching, and clinic, 668 matched blastocysts were analyzed (334 derived from in vitro–matured MI oocytes vs 334 derived from in vivo–matured MII oocytes). Participants/materials, setting, methods The study compared blastocysts from MI oocytes matured in vitro and injected after first polar body extrusion (confirmed MII at ICSI) with blastocysts from oocytes already at MII at retrieval. Outcomes included blastocyst development day, expansion grade (Gardner), inner cell mass, and trophectoderm quality. Paired analyses used McNemar and Wilcoxon tests, with conditional and multinomial logistic regression adjusting for matching and age. Main results and the role of chance Blastocysts derived from MI oocytes matured in vitro showed a higher proportion of early expansion stages (Gardner grade 1) compared with those derived from in vivo matured MII oocytes (12.0% vs 7.8%; adjusted OR [aOR] 1.7, 95% CI 1.0–2.9; p = 0.064). They also had a lower probability of achieving advanced expansion (≤3 vs > 3; aOR 1.5, 95% CI 1.0–2.3; p = 0.041). No significant differences were observed in blastulation timing (day 6 vs day 5; aOR 1.2, 95% CI 0.9–1.6; p = 0.358). Poor-quality ICM and TE (grade D) were more frequent among blastocysts derived from MI oocytes. Blastocysts from in vivo matured MII oocytes showed a significantly higher frequency of TE grade A (aOR 1.7, 95% CI 1.2–1.3; p = 0.006), while no significant differences were observed in ICM grade A. Limitations, reasons for caution The retrospective design limits causal inference, and embryo competence was assessed only by morphological parameters. Clinical outcomes such as implantation, pregnancy or live birth were not evaluated. Wider implications of the findings Although MI oocytes matured in vitro can generate blastocysts, differences in expansion stage and trophectoderm quality should be considered during embryo selection and patient counselling in clinical practice. Trial registration number No

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