Objective: To determine the optimal leading follicle size for triggering final oocyte maturation in POSEIDON groups 3 and 4 poor responders undergoing ART cycles. Design: Retrospective cohort study. Setting: University based Infertility Centre. Population: Data of 294 POSEIDON groups 3 and 4 poor responders aged between 20 and 42 years who underwent ICSI following a GnRH antagonist cycle between January 2015 and July 2021 were reviewed. Methods: Among the 342 patients eligible in our database, 294 fulfilling inclusion criteria were assessed for final analyses. Cycles were categorized into two groups according to occurrence of premature ovulation. Premature ovulation was defined as visualization of rupture of at least one of the leading follicles on the day of oocyte retrieval. In addition, number of oocytes retrieved, number of MII oocytes, MII/antral follicle count (AFC) ratio and follicle-oocyte index (FOI) were compared between different leading follicle sizes. Main Outcome Measures: Number of oocytes retrieved, number of MII oocytes, MII/antral follicle count (AFC) ratio and follicle-oocyte index (FOI). Results: Among all, 47 (16.2%) had premature ovulation between the trigger and oocyte pick-up days. The mean size of the leading follicle on the day of trigger was significantly higher in the premature ovulation group than the controls (19.8±2.4 mm vs.18.7±2 mm, respectively; P<0.001). Multivariate logistic regression analyses identified baseline LH (Odds ratio {OR} 1.144, 95% confidence interval {CI} 1.052-1.243; P=0.002), number of follicles >11 mm on the day of trigger (OR 0.580, 95% CI 0.438-0.767; P<0.001), and leading follicle size (OR 1.361, 95% CI 1.130-1.641; P=0.001) as independent predictors of premature ovulation. According to the one-way ANOVA test and non-linear curve estimation model the FOI and MII/AFC ratios peaked when the leading follicle size was between 16-17 mm, respectively. Conclusion: Individualized trigger based on leading follicle size can provide maximum efficiency in ovarian stimulation in POSEIDON expected poor responders. While late trigger may result in premature ovulation, early trigger may also result in less MII. Triggering when the leading follicle size is between 16.5 and 17 mm can help to prevent these negative outcomes and achieve optimal cycle outcome.