Each egg cell (oocyte) is contained within a small structure called a follicle, with each follicle holding just one oocyte. Women are born with a fixed number of follicles—and therefore eggs—which cannot regenerate or be replenished. This means that from birth, a woman has a lifelong supply that gradually declines over time. Most of these follicles remain in a "dormant" state, and throughout her life, she continuously loses eggs, roughly one per month. This natural depletion leads to a gradual decline in fertility, with peak fertility occurring between ages 20 and 30. From around 35-37 years old, fertility declines more rapidly, not only due to a decrease in the number of eggs but also because of a reduction in their quality.  

At birth, a woman has about 1 million follicles, but by puberty, this number drops to 300,000 to 400,000. Over her reproductive years, she will ovulate around 400 times until menopause.  

Throughout life, a limited number of follicles leave the dormant reserve and begin the complex maturation process known as folliculogenesis. Each month, only a small group of these follicles reaches a partially mature stage. Among them, a select few, known as antral follicles, compete to complete the final maturation phase. Ultimately, only one follicle fully matures and releases an egg—a process called ovulation. All other follicles that started maturing but did not reach ovulation are naturally eliminated. This means that less than 1% of a woman's total follicle supply is ever ovulated.