Non-linear impact of abnormal fertilization on sibling 2PN embryo development: a 107,527-embryo cohort study defining...

Abnormal fertilization is a frequent occurrence in in vitro fertilization (IVF) cycles, yet the impact of abnormally fertilized zygotes on their sibling normally fertilized (2PN) embryos remains a subject of ongoing debate. This study aimed to characterize the non-linear relationship between the abnormal fertilization rate (AFR) and sibling 2PN blastocyst developmental potential to identify a clinically actionable biomarker for oocyte cohort quality.

We conducted a large-scale retrospective cohort study analyzing 107,527 sibling 2PN-derived embryos from 13,167 conventional IVF cycles (March 2019-January 2023). AFR was defined as the proportion of non-2PN zygotes among all fertilized oocytes. Restricted cubic splines (RCS) were employed to explore potential non-linearity, and multivariable generalized estimating equation (GEE) models were used to assess the independent impact of AFR on blastocyst formation, adjusting for 22 clinical and embryological confounders while accounting for embryonic clustering.

RCS analysis identified a critical biological tipping point at 25% AFR. Beyond this threshold, the odds of blastocyst formation for sibling 2PN embryos significantly declined (adjusted odds ratio (aOR) 0.933, 95% CI 0.883-0.986, P = 0.014). Subgroup analysis demonstrated that this developmental impairment was robustly preserved in patients with adequate ovarian reserve (AMH ≥ 1.2 ng/mL: aOR 0.934, P = 0.018) and standard oocyte yields (5-14 oocytes: aOR 0.913, P = 0.017). Conversely, the association was not statistically significant in ultra-high-yield cycles (≥ 15 oocytes) or diminished ovarian reserve (AMH < 1.2 ng/mL), which may be primarily attributed to the truncated sample sizes and diminished statistical power within these specific sub-cohorts.

Our findings establish AFR as a sensitive, independent biomarker of oocyte cohort integrity. The identification of the 25% threshold provides a high-level evidence base for Day-1 risk stratification, personalized prognostic counseling, and optimized laboratory quality management. Integrating this tool into ART workflows can enhance the precision of clinical outcome expectations.