Protocol-Specific Luteinizing Hormone Thresholds for IVF Pregnancy Outcomes: The Moderating Role of Age in a Retrospective Study of 217 Cycles

Objective: To explore the impact of trigger-day luteinizing hormone (LH) levels on pregnancy outcomes in antagonist and long protocols during in vitro fertilization (IVF) and determine protocol-specific LH thresholds. Methods: This retrospective analysis covered 217 IVF/ICSI cycles from January 2023 to April 2024. The relationships between trigger-day LH levels and pregnancy outcomes in the two protocols were compared using ROC curves, multivariate logistic regression, and mediation models. Results: In the antagonist protocol, LH > 1.45 IU/L was linked to lower oocyte yield, reduced high-quality embryo rates, and a higher risk of clinical pregnancy failure (RR = 2.34, p = 0.006). In the long protocol, LH > 1.0 IU/L was associated with higher clinical pregnancy rates (OR = 2.61, p = 0.026), with 68% of the effect mediated by increased oocyte yield. Age ≤ 35 significantly enhanced these effects (three-way interaction OR = 6.55, p = 0.003). Conclusion: The antagonist and long protocols should adopt LH thresholds of 1.45 IU/L and 1.0 IU/L, respectively. Age is a key moderator, providing evidence for individualized IVF trigger strategies. Keywords: luteinizing hormone, antagonist protocol, long protocol, in vitro fertilization, clinical pregnancy, age interaction