Females are born with all the eggs they will ever have, a fundamental aspect of human reproduction. Understanding this biological process clarifies the unique timeline of female fertility. The development of eggs and the dynamics of the ovarian reserve shape a woman’s reproductive lifespan.
The Genesis of Eggs
Female humans are born with a finite number of egg cells, a supply established before birth. Egg formation, known as oogenesis, begins early during a female fetus’s development. At approximately 20 weeks of gestation, a female fetus has the highest number of potential egg cells, estimated at 6 to 7 million. This number significantly decreases even before birth, with a newborn girl typically having about 1 to 2 million immature egg cells, called oocytes, stored in her ovaries. No new egg cells are generated after birth.
The Ovarian Reserve Over Time
The fixed number of oocytes continues to decline throughout a female’s life, a process that accelerates over time. This reduction occurs primarily through a natural process called atresia, which is a form of programmed cell death where follicles degenerate. While a small number of oocytes mature and are released through ovulation each month, the vast majority are lost to atresia. By the time a girl reaches puberty, her ovarian reserve has typically decreased to about 300,000 to 400,000 oocytes. This continuous decline progresses throughout adulthood, eventually leading to menopause, when the ovarian reserve is nearly depleted, often with around 1,000 follicles remaining.
Understanding Fertility’s Timeline
The finite and continuously declining supply of eggs has direct implications for a woman’s reproductive timeline. Both the quantity and quality of a woman’s eggs naturally diminish with age. While fertility gradually declines from the late teens, the rate of decline often accelerates significantly after the mid-30s. For instance, the chance of pregnancy per cycle can drop considerably for women over 35. Beyond a decrease in numbers, older eggs are more prone to genetic abnormalities, which can impact the ability to conceive and increase the risk of miscarriage.
A Distinct Reproductive Strategy
The female reproductive strategy, characterized by a finite egg supply established before birth, contrasts with male reproductive biology. Males continuously produce new sperm throughout their adult lives, a process known as spermatogenesis. This continuous production generates hundreds of millions of sperm cells daily. The female system operates with a pre-determined, non-renewable reserve, highlighting a fundamental biological difference in how each sex approaches reproduction.