The correlation between follicular measurements, oocyte morphology, and fertilization rates in an in vitro fertilization program

OBJECTIVE: To explore the relationship between follicle size and the morphology of the oocyte-cumulus-corona complex with fertilization rates in stimulated cycles of IVF. DESIGN: Retrospective comparison of measurements and observations of 2,429 oocytes from 215 patients undergoing 324 stimulated IVF cycles. SETTING: A large hospital-based IVF program. MAIN OUTCOME MEASURES: Individual follicles were measured by ultrasound before transvaginal aspiration and the size was recorded. The oocyte-cumulus-corona complex from each follicle was examined and classified. The oocytes were checked for evidence of fertilization 17 to 22 hours after insemination. RESULTS: The fertilization rate of all oocytes regardless of morphological type revealed a positive linear correlation with increasing follicle diameter. The fertilization rates of type I oocytes was marginally higher than type II oocytes, controlling for follicle diameter; however, this difference did not achieve statistical significance. Oocytes from follicles with a mean diameter > or = 16 mm had significantly higher fertilization rates than did oocytes from follicles with a mean diameter < or = 14 mm. CONCLUSIONS: Follicle size is a better predictor of fertilization than is morphological characterization of the oocyte-cumulus-corona complex in IVF.