The Trigger in IVF Cycles: Molecular Pathways and Clinical Implications

The final trigger of oocyte maturation is a pivotal step in assisted reproductive technology (ART). Different molecules and protocols—including human chorionic gonadotropin (hCG), gonadotropin-releasing hormone agonists (GnRHa), the dual trigger, the double trigger, and emerging agents such as kisspeptin—have been investigated to optimize oocyte competence, embryo development, and pregnancy outcomes while minimizing the risk of ovarian hyperstimulation syndrome (OHSS). HCG remains the most widely used trigger, but its pharmacological profile is associated with a significant risk of OHSS. GnRHa has emerged as an alternative in antagonist cycles, abolishing the risk of severe OHSS but often requiring tailored luteal phase support. Several strategies, including hCG, GnRHa, and combined approaches, have shown improvements in specific outcomes such as the oocyte maturity (MII) rate, fertilization rate, embryo development parameters, and, in selected contexts, a reduction in OHSS risk. Kisspeptin represents a promising option; however, its use remains predominantly within the research setting, with clinical application still limited to early-phase or highly selected studies. Beyond the choice of molecule, the timing of trigger administration—adjusted to follicle size, estradiol concentrations, and progesterone levels—also influences oocyte competence and subsequent clinical outcomes. Triggering final oocyte maturation remains a multifaceted decision that should be individualized according to patient characteristics, ovarian response, and risk of OHSS. Although hCG remains the historical reference standard, accumulating but heterogeneous evidence suggests that GnRHa-based strategies, including dual-trigger protocols, may improve specific outcomes in selected patient subgroups. However, results across trials are inconsistent, particularly in poor responders, and any exposure to hCG maintains a residual risk of OHSS. Kisspeptin represents a promising but still experimental option, with current data largely limited to early-phase clinical studies in highly selected high-risk populations. Well-designed randomized trials are required to clarify the true impact of these strategies on live birth, to refine timing and dosing, and to better define which patients are most likely to benefit.