The association between follicle size and oocyte development as a function of final follicular maturation triggering

Research question: To study the association between follicle size and oocyte/embryo quality, as a function of different triggering modes for final follicular maturation. Study design: Cohort study conducted in a single tertiary medical centre between July 2018 and May 2019. All women undergoing ovarian stimulation with triggering using human chorionic gonadotrophin (HCG), gonadotrophin-releasing hormone (GnRH) agonist or dual trigger (GnRHa + HCG) were included. Before ultrasound-guided follicular aspiration, follicles were measured and divided into three groups according to maximum dimensions: large ≥16 mm, medium 13–15 mm and small <13 mm. Microscopic examination of the follicular aspirates was performed by an embryologist. Each follicle aspirated was evaluated for oocyte maturation, oocyte fertilization and embryo quality. Results: A total of 640 follicles were measured, including 402 (62.8%) in the large, 148 (23.1%) in the medium and 90 (14.1%) in the small groups. Oocytes were obtained during aspiration from 76.3%, 70.3% and 55.6% of the large, medium and small follicle groups, respectively (P = 0.001). The mature oocyte (metaphase II) rate was significantly higher in the large (P = 0.001) and medium (P = 0.01) compared with the small follicle group. Nevertheless, no between-group differences were observed in fertilization or top quality embryo rates among mature oocytes regardless of the size of the follicle from which they originated. Triggering mode did not influence oocyte recovery rate in the different follicle size groups. Conclusion: A higher oocyte recovery rate was observed from follicles >13 mm, however, mature oocytes achieved similar fertilization and top quality embryo rates regardless of follicle size. Triggering mode did not influence oocyte recovery rate.