Nonlinear effects of post-denudation timing on day 3 embryo outcomes in ICSI and evidence for a translatable...

Intracytoplasmic sperm injection (ICSI) is performed in over half of assisted reproductive technology cycles globally; however, the optimal interval between oocyte denudation and sperm injection remains undefined in clinical guidelines. This knowledge gap hinders the standardization of laboratory workflows and may compromise embryo developmental potential due to suboptimal timing practices. It necessitates a comprehensive investigation to formulate evidence-based recommendations.

This retrospective cohort study examined 1,152 consecutive fresh ICSI cycles performed at an academic reproductive center between January 2024 and April 2025 to establish quantitative timing guidance. The denudation-to-ICSI interval (DTI) was defined as the primary exposure, and the day-3 embryo utilization rate, calculated as the number of usable embryos divided by the number of retrieved oocytes, was designated as the primary outcome. Nonlinear dose-response relationships were evaluated using generalized additive models and restricted cubic spline analyses, with multivariable adjustment for demographic characteristics, ovarian reserve parameters, and treatment-related factors. Effect heterogeneity was examined across 19 predefined patient subgroups, and robustness of the findings was validated through bootstrap resampling and cross-validation.

A significant inverted U-shaped relationship was identified between DTI and day 3 embryo utilization rate. In the final parsimonious model adjusted for basal FSH and E2 per MII (p = 0.017), predicted utilization rates spanned 12.2 percentage points across the observed DTI range, peaking at 2.75 h post-denudation and defining an optimal window of 1.75-3.75 h. This association was robust to progressive covariate adjustment (24% coefficient attenuation from univariate to fully adjusted model, p = 0.036) and consistent across all 19 patient subgroups (no significant interactions; I² < 35%). The timing effect was specific to day 3 embryo development without influencing fertilization or cleavage rates. Approximately 30% of current cycles fell outside the optimal window, corresponding to 4.6 percentage points lower utilization.

This study identified an optimal window of 1.75-3.75 h for ICSI after oocyte denudation, with consistent associations observed across diverse patient subgroups within the studied population. The identified window represents a workflow-level optimization achievable through procedural standardization alone, with the potential to increase embryo availability for a substantial proportion of cycles currently outside this timeframe. Future trials are warranted to confirm causality and measure improvements in clinical pregnancy outcomes.

Not applicable.