Women are born with a finite and non-renewable supply of eggs, housed within follicles in the ovaries. These cells represent a woman’s entire reproductive potential. The number of these eggs gradually diminishes over time, influencing fertility and ultimately leading to menopause.
How Many Eggs Are Women Born With?
Women are born with their complete lifetime reserve of eggs, established during fetal development. At birth, a female typically has between 1 and 2 million eggs, each encased within a primordial follicle in her ovaries. This number begins to decline even before puberty. By the onset of menstruation, the egg count has significantly reduced, often to a range of 300,000 to 500,000 eggs. No new eggs are generated after birth.
How Eggs Are Lost Each Month
A woman’s egg supply continuously reduces through two primary mechanisms: atresia and ovulation. While typically only one egg is released during ovulation each menstrual cycle, many more are lost through atresia. Atresia is programmed cell death where ovarian follicles, containing immature eggs, degenerate and are reabsorbed.
Each month, a group of several hundred to a thousand immature eggs begins to develop. From this group, usually only one follicle matures fully and releases its egg during ovulation. The remaining eggs in that group, which do not reach full maturity, undergo atresia. This depletion occurs irrespective of factors like pregnancy or birth control.
What Affects the Rate of Egg Loss?
The rate of egg loss is primarily influenced by biological aging. The decline in egg count accelerates as a woman ages, becoming more pronounced after the mid-30s. This age-related decrease affects both the quantity and quality of eggs.
Genetic factors also play a role; some women may have a predisposition to a faster decline in ovarian reserve or early menopause. Certain medical treatments, including chemotherapy, radiation therapies, and some ovarian surgeries, can significantly impact egg supply. Lifestyle choices, such as smoking, are linked to an accelerated rate of egg loss and can lead to earlier menopause.
How Egg Loss Affects Fertility
The ongoing loss of eggs directly impacts a woman’s reproductive capacity, leading to diminished ovarian reserve. This reduces both the quantity and quality of remaining eggs. As ovarian reserve diminishes, the chances of natural conception decrease.
The quality of remaining eggs also declines with age. Older eggs may have a higher likelihood of genetic abnormalities, which can reduce successful fertilization and implantation, and increase miscarriage risk. Menopause is the culmination of this egg depletion, occurring when the supply of viable eggs becomes critically low, typically fewer than 1,000.