2.5. Collection and preservation of follicular fluid
About 35 to 36 hours after the injection of hCG, a B-ultrasound-guided vaginal puncture was performed to extract the egg, and the first tube of bloodless pale yellow clear follicular fluid was collected. After confirming the phase of MII oocytes under the microscope, the collected follicular fluid was centrifuged at 3000g for 15 minutes. The upper fluid was aliquoted into Eppendorf tubes marked with the name of the patient and the date of egg retrieval and directly frozen in the −80°C cryogenic refrigerator for later use.