The dilemma of the trigger timing in IVF: a review

Triggering final oocyte maturation is a pivotal step in modern patient-tailored IVF/ICSI treatment, securing the optimal number of mature oocytes retrieved without compromising fertilization, embryo development, and live birth. Several factors need to be considered when deciding the time of the trigger: the size of the leading follicles, distribution of the follicular cohort, the duration of stimulation, the protocol used for stimulation, and ovarian response status. The current narrative review aims to appraise all available evidence for determining the proper time for inducing final oocyte maturation following IVF treatment. Moreover, it discusses the impact of the stimulation protocol, follicular size, and magnitude of ovarian response on choosing the proper timing for trigger. Comprehensive literature search of all available articles and relevant articles studying the criteria for timing of final oocyte maturation trigger in IVF/ICSI cycles were included in this review. It was found that leading follicles size of 16–22 mm is associated with the optimum oocyte maturation ratio, size of the remaining cohort of follicles should be ≥ 14 mm, 10–12 days of minimum length of stimulation should be auspicated in normal responders before trigger, and the timing of trigger administration should not depend solely on hormonal levels. In conclusion, the timing of triggering of final oocyte maturation in ICSI cycles should be individualized on a case-by-case basis.