Received November 30, -0001
Accepted September 14, 2020

Objective: The aim of this study was to evaluate the fertilization and blastocyst formation rates of oocytes in metaphase I (MI) obtained from women who underwent controlled ovarian hyperstimulation (COH) for intracytoplasmic injection. Methods: A prospective cohort study that included women from whom at least 1 MI and 1 MII oocyte were obtained after COH was performed. We collected 1907 oocytes from 164 women (1291 MII, 352 MI and 258 prophase I or atresic). After oo-cyte classification, the MII and MI oocytes were incubated for 4 hours. Results: After 4 hours, the rescue maturation rate was 57.2%; 205 MI oocytes matured to MII oocytes in vitro (rescued MI-MII group), and 153 remained in MI (ar-rested MI group). The normal fertilization rates were directly associated with oocyte maturation, with rates of 79.1%, 60.2%, and 31.9% in MII, MI-MII and MI oocytes, respectively (P <0.001). Group arrested MI had an odds ratio (OR) of 7.6 (CI 5.2 - 11.2, P <0.001) for abnormal fertilization compared with Group MII. The blastocyst for-mation rate was directly associated with oocyte maturation, at 36.4% for MII, 11.4% for MI-MII and 0.6% for MI. Conclusion: Oocytes collected at the MI stage after OCH that do not mature to MII after rescue maturation have a blastocyst formation rate of only 0.6%, while those in MII and MI-MII have rates of 36.4% and 11.4% respectively. However, we ob-served a pregnancy with the birth of a healthy baby from a blastocyst formed after in-tracytoplasmic sperm injection (ICSI) of an MI oocyte.