The question of whether a person is born with all the eggs they will ever have is fundamental in reproductive biology. The traditional and widely accepted answer is largely yes, pointing to a finite, non-renewable supply established early in development. This fixed supply, known as the ovarian reserve, dictates the length and potential of a person’s reproductive lifespan. Understanding this reserve requires examining the biological processes that occur before birth and the continuous loss that follows.
The Formation of the Initial Supply
The process that creates the initial stock of egg cells, called oogenesis, begins while the female is still a fetus inside the womb. Specialized cells, known as primordial germ cells, migrate to the developing ovaries and rapidly multiply through mitosis. This intense cell division establishes the maximum number of germ cells a person will ever possess.
The peak number of germ cells is reached around the middle of gestation, approximately the fifth month, often totaling six to seven million. Following this peak, programmed cell death significantly reduces this number before birth. The surviving cells then enter the first phase of meiosis, a type of cell division that reduces the chromosome number.
These primary egg cells, or primary oocytes, halt their development midway through the first meiotic division, entering meiotic arrest. They remain suspended in this state, encased within protective structures called primordial follicles, for potentially decades. By the time of birth, the total number of oocytes has dropped to roughly one to two million, and by the onset of puberty, only about 300,000 to 400,000 remain.
The Continuous Decline of the Egg Reserve
Once the initial supply is established in the fetus, the number of follicles containing oocytes begins a continuous decline that lasts until menopause. This reduction of the ovarian reserve is driven by a natural biological process known as follicular atresia. Atresia is a programmed mechanism that causes the death and reabsorption of the vast majority of follicles.
The process occurs independent of external factors like hormonal contraceptives, general health, or pregnancy. For every single egg that is successfully matured and ovulated, hundreds to thousands of other follicles are lost through atresia. It is estimated that approximately 99.9% of all oocytes present at the fetal peak are eventually eliminated by this mechanism before reproductive life ends.
Atresia ensures that only the healthiest follicles, containing the most viable oocytes, are selected for maturation each month. This follicular loss occurs across all stages of development, from the primordial stage to the larger, mature follicles. This programmed cell death contributes to the fact that only about 300 to 400 eggs will ever be released through ovulation over a person’s reproductive lifetime.
The rate of atresia is not constant throughout life but accelerates significantly in the years leading up to menopause. By the time menopause is reached, typically around age 51, the ovarian reserve is depleted to less than 1,000 residual follicles. This exhaustion of the fixed supply of follicles signals the end of reproductive capacity.
How Age Affects Reproductive Potential
The decline of the ovarian reserve affects fertility in two ways: a decrease in the quantity of available eggs and a decline in their quality. While the reduction in numbers makes it more difficult to conceive, the drop in oocyte quality is the main factor driving age-related infertility. This decline begins to accelerate noticeably after the mid-30s.
The most significant age-related issue is the increased risk of aneuploidy, which refers to an incorrect number of chromosomes in the egg cell. These chromosomal errors occur because the egg has been suspended in meiotic arrest for a prolonged period, leading to structural and molecular impairments in the cellular machinery. The longer the egg remains dormant, the higher the chance that the spindle apparatus, which separates chromosomes during cell division, will malfunction.
Increased aneuploidy results in a higher incidence of miscarriages and a lower chance of success with assisted reproductive technologies. Additionally, aging oocytes can suffer from mitochondrial dysfunction, which reduces the energy available for successful fertilization and early embryo development. The combination of dwindling numbers and declining cellular health limits reproductive potential with advancing age.
The Current Scientific Debate on New Egg Creation
The long-held biological view of a fixed egg supply has faced challenges from a controversial area of research over the past two decades. Some studies have suggested the potential existence of Ovarian Stem Cells (OSCs) in adult mammalian ovaries. These cells, if functional, would theoretically be capable of dividing and generating new oocytes throughout life, contradicting established dogma.
However, the findings supporting the existence of OSCs are not widely accepted and remain highly debated. Other research groups have used advanced techniques, such as single-cell analysis, to map the cell types within the human ovary. These later studies concluded that the cells previously identified as egg stem cells may actually be perivascular cells or cellular artifacts created during laboratory culture.
The consensus remains that, for practical clinical purposes, the human ovarian reserve is finite and established before birth. While the possibility of discovering a mechanism for adult oogenesis continues to be researched, no current fertility treatments are based on creating new eggs from stem cells. The primary focus of clinical fertility preservation remains on conserving the existing, aging supply.