Impact of follicular size categories on oocyte quality at trigger day undergoing luteal phase stimulation protocol.

Purpose: To investigate an optimal strategy by assessing the effectiveness of varying follicular sizes on trigger day during luteal phase stimulation protocol and provide evidence for personalized protocol adjustment.

Methods: This was a retrospective study including a total of 661 patients who had started their in vitro fertilization cycle with a luteal phase stimulation (LPS) protocol during 2015-2023. We classified patients into groups according to the size of the dominant proportion of follicles on the human chorionic gonadotropin (hCG) trigger day: large, medium, and small. The metaphase II (MII) oocyte rate, immature oocyte rate, two pronuclei (2PN) fertilization rate, and available embryo rate were compared between groups. General linear model (GLM) analysis was performed for inter-group comparison of the oocyte and embryo quality.

Results: There was a statistically significant difference in the immature oocyte rate between the three groups (p = 0.005), with the Large group having the lowest immature oocyte rate and the Small group having the highest immature oocyte rate, and there was no difference in MII oocyte rate among the three groups. As for the 2PN fertilization rate, there was a statistical difference among the three groups, with the highest 2PN fertilization rate in the Large group and the lowest in the Small group. However, there was no statistically significant difference in the available embryo rate among the three groups. The GLM analysis suggested that different follicle sizes on the trigger day influenced the immature oocyte rate and the 2PN fertilization rate. But among the young women, applying different follicle sizes on the trigger day of the LPS regimen did not influence the oocyte and embryo quality, while in older women it affected the 2PN fertilization rate.

Conclusion: Triggering when there is a high proportion of large follicles results in lower immature oocyte rate and higher 2PN fertilization rate when applying LPS protocol. However, age is a factor to be considered and the timing of triggering needs to be decided in clinical application with consideration of age and the actual situation.