Oocyte (Egg)

An oocyte is the female gamete. Women are born with all the oocytes they will ever have — roughly 1–2 million at birth, declining to 300,000–500,000 at puberty. Only about 400 are ever ovulated; the rest undergo atresia. During each menstrual cycle, a cohort of follicles is recruited, but normally only one (the dominant follicle) reaches full maturity.

In IVF, gonadotropins recruit multiple follicles. Retrieved oocytes are classified by maturity: germinal vesicle (GV), metaphase I (MI), or metaphase II (MII — fully mature). Only MII eggs can be fertilized. Egg quality — influenced heavily by maternal age — is the single most important factor in IVF success.