The ovaries are vital organs in the female reproductive system. These small, thumb-sized organs are located in the pelvis, attached to each side of the uterus, near the fallopian tubes. Within these ovaries lies a woman’s supply of oocytes, commonly known as eggs, the female reproductive cells.
From birth, females possess a predetermined number of eggs, a concept that is fundamental to understanding women’s fertility and reproductive lifespan. At birth, a female baby typically has approximately 1 to 2 million eggs in her ovaries. This is the total supply she will ever have, as women cannot produce new eggs throughout their lives. This finite number is a key aspect of female reproductive biology, setting it apart from sperm production in males.
However, this substantial number of eggs present at birth steadily declines throughout a woman’s life. By the time a girl reaches puberty, a significant portion of her initial egg supply has already diminished. It’s estimated that only around 25% of her original egg count remains, leaving approximately 300,000 eggs. This reduction is a natural biological process, and the decline continues throughout a woman’s reproductive years.
Over the subsequent 30 to 40 years of a woman’s reproductive life, this egg supply will continue to decrease. While the precise number of eggs a woman has at any given time cannot be determined with certainty, a noticeable decrease in fertility often begins around the age of 37. As a woman approaches menopause, the number of remaining eggs becomes minimal. Menopause marks the end of a woman’s reproductive years, occurring when the ovaries have virtually depleted their egg supply.
It’s important to note that the vast majority of eggs present in the ovaries are immature, known as primordial follicles. Each month, a number of these follicles are recruited to begin the process of growth and maturation. However, only a small fraction will fully mature. The majority of these developing follicles and oocytes will undergo a process called atresia, essentially programmed cell death. In fact, it is estimated that only about 300 to 500 eggs will actually mature and be released through ovulation during a woman’s entire reproductive lifespan.
The maturation of an egg is a process that typically spans around 14 days and can be divided into two phases. Initially, many eggs, sometimes as many as a thousand, start to develop. The second phase requires hormonal signals from gonadal hormones to further stimulate development. Despite numerous eggs beginning maturation, usually only one egg becomes dominant each menstrual cycle and reaches full maturity, ready for ovulation and potential fertilization. The remaining eggs and follicles that were developing will then degenerate. This monthly process of recruiting and losing eggs contributes to the continuous decline in a woman’s total egg pool from birth until menopause.
In women who have gone through puberty, the dominant follicle and egg continue to mature under the influence of hormones. When the egg is fully mature, the follicle ruptures, releasing the mature egg into the fallopian tube, on its path towards the uterus. The egg is then capable of being fertilized for a period of approximately 48 hours. If fertilization does not occur within this timeframe, the egg will die, and in roughly two weeks, a new cycle of egg maturation will commence.
This cyclical process of egg development repeats throughout a woman’s life until the egg supply is largely exhausted, leading to menopause. Menopause typically occurs in a woman’s 40s or 50s, with the average age in the United States being around 51. The depletion of the egg pool before the age of 40 is considered premature ovarian failure. Furthermore, treatments like chemotherapy, which can damage ovarian follicles, can also increase the risk of premature ovarian failure, even years after the treatment has concluded. This is an important consideration for family planning, especially for young girls undergoing such treatments.
Understanding the finite nature of a woman’s egg supply and the process of egg maturation is crucial for comprehending female fertility, reproductive health, and the biological changes associated with aging.
