Abstract / Summary
Abstract Background Conventional in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), are mainstream fertilization strategies for assisted reproduction. Different fertilization strategies adopt distinct timelines for cumulus cell removal and oocyte maturity assessment. Oocyte nuclear maturity is critical for fertilization and embryonic development. Apparent inter group differences in oocyte maturity may be driven by in vitro post insemination maturation of immature oocytes during prolonged culture. Objective This study aimed to disentangle the contribution of fertilization strategy and assessment timing to observed oocyte maturity, and to explore corresponding clinical assisted reproductive treatment (ART) outcomes. Methods A retrospective cohort study was conducted on patients undergoing their first fresh ART cycles at the Reproductive Medicine Center of Shijiazhuang Fourth Hospital between January 2020 and December 2024. Patients were stratified according to fertilization strategy: short-term fertilization ( n = 958 cycles), overnight fertilization ( n = 467 cycles), ICSI ( n = 404 cycles), and rescue-ICSI ( n = 230 cycles). Cycles containing oocytes with empty zona pellucida or degenerated morphology were excluded. Only cycles with oocytes suitable for meiotic-status evaluation were enrolled. Generalized estimating equation (GEE) logistic regression was applied to handle oocyte clustering within individual ovarian stimulation cycles. Multivariable models were adjusted for female age, BMI, number of retrieved oocytes, infertility type, ovarian stimulation protocol, and basal reproductive hormone parameters. For rescue-ICSI cycles with complete laboratory timestamps, subgroup analysis was performed stratified by rescue timing. Clinical pregnancy related analyses were restricted to cycles with completed embryo transfer and follow-up. Multivariable binary logistic regression was used for cycle level pregnancy endpoints. P < 0.05 was considered statistically significant. Results In the short-term fertilization cohort, paired day 0 (D0) and day 1 (D1) observation demonstrated obvious in vitro oocyte maturation: 774 non-MII oocytes at D0 progressed to MII status after overnight culture (McNemar χ² = 774.000, OR = 1.620, 95%CI : 1.540–1.710, P < 0.001). After adjustment for confounders and matching observation time points, at D0, the ICSI group showed higher odds of MII maturity (adjusted OR = 1.690, 95%CI : 1.460–1.960, P < 0.001), whereas rescue-ICSI presented lower MII odds (adjusted OR = 0.820, 95%CI : 0.710–0.940, P = 0.004). At D1, overnight fertilization was associated with higher MII probability compared with short-term fertilization (adjusted OR = 1.310, 95%CI : 1.100–1.550, P = 0.003). Significant inter group differences were found in 2PN fertilization rate and blastocyst formation rate (both P < 0.001). In rescue-ICSI subgroup analysis stratified by actual laboratory timestamps, no significant difference in 2PN fertilization rate was observed between early and late subgroups, and no significant improvement in clinical reproductive endpoints was observed. After multivariable adjustment, no independent association was detected between fertilization modality and clinical pregnancy or live birth outcomes (all P > 0.05). Conclusion Immature oocytes can complete meiotic maturation during extended incubation, which increases the proportion of observed mature oocytes at later assessment time points. Although significant differences in oocyte level maturity and laboratory embryological indices exist among fertilization groups, these differences were not translated into independent improvements in clinical pregnancy or live birth in the present cohort. Clinical trial number Not applicable.