The reproductive potential of an individual with ovaries is defined by the ovarian follicle. This small biological unit contains the immature egg cell and the surrounding support structure necessary for reproduction. The quantity of these follicles is established before birth and is finite. This supply undergoes a continuous, natural decline throughout life. Understanding the number of these follicles and their loss provides insight into the biological timeline of fertility.
What Exactly is an Ovarian Follicle?
An ovarian follicle is a roughly spherical cellular aggregation found within the ovaries. It is a fluid-filled sac that contains and nourishes a single, immature egg, known as an oocyte. The follicle is composed of the oocyte, granulosa cells, and theca cells, which support the egg’s development. Follicles also produce hormones, such as estrogen, necessary for regulating the menstrual cycle.
Follicles progress through several distinct stages of development. They begin as dormant primordial follicles, present at birth, which can be activated to become primary, secondary, and eventually tertiary or antral follicles. The antral stage is characterized by the formation of a fluid-filled cavity, the antrum. This stage represents the final developmental step before ovulation, though only a tiny fraction of all follicles ever reach it.
The Quantitative Count: Follicles by Age Group
The maximum number of follicles an individual will ever possess is reached during fetal development, between 18 and 22 weeks of gestation. At this peak, the total number of non-growing follicles across both ovaries is estimated to be six to seven million. This peak count is immediately followed by the beginning of a continuous decline that lasts the entire lifespan.
By the time of birth, the total number of follicles remaining in both ovaries has dropped significantly, typically ranging from one to two million. This provides a pool of approximately 500,000 to one million follicles per ovary at the start of life. The decline continues steadily, meaning an individual reaches puberty with a greatly reduced supply.
Upon reaching reproductive maturity, the total follicle count has decreased further, settling in the range of 300,000 to 500,000 across both ovaries. This remaining count represents the entire reserve available for the reproductive years. By the time an individual reaches menopause, the ovarian reserve is nearly exhausted, often with fewer than 1,000 follicles remaining. These figures represent average estimates, and significant variation exists between individuals.
Natural Attrition: The Lifelong Decline
The steep decrease in follicle numbers from the fetal peak to puberty is primarily driven by a continuous biological process called atresia. Atresia is the degeneration of ovarian follicles, accounting for the loss of over 99% of the follicles an individual is born with. This process is relentless and occurs at all stages of follicular development, from the dormant primordial stage to the larger antral stage.
Each menstrual cycle, a cohort of small follicles begins to grow, but typically only one is selected to mature fully and ovulate. All other follicles in that cohort are lost to atresia. The mechanism of atresia is hormonally controlled, involving the apoptosis of the supporting granulosa cells within the follicle.
Factors such as genetics and environmental exposures can influence the rate of attrition. The rate of follicular loss accelerates significantly in the years leading up to menopause. This acceleration often occurs when the total number of follicles falls below a certain threshold, marking the final reproductive stages before the exhaustion of the ovarian reserve.
Measuring Ovarian Reserve
Because the actual number of primordial follicles is impossible to count directly, medical professionals estimate the remaining follicle supply, known as the ovarian reserve. One common method involves a blood test to measure the level of Anti-Müllerian Hormone (AMH). AMH is a hormone produced by the small, growing follicles within the ovaries.
The level of AMH detected in the blood directly correlates with the overall number of small follicles present in the ovaries. As the follicle reserve declines with age, AMH production also decreases, making it a reliable indicator of the remaining supply. This test can be performed at any point in the menstrual cycle because AMH levels remain relatively stable.
Another widely used method is the Antral Follicle Count (AFC), performed using a transvaginal ultrasound. The AFC involves counting the total number of small, fluid-filled antral follicles, typically 2 to 10 millimeters in size, visible in both ovaries. The number of these visible follicles provides a real-time estimate of the follicles available for that particular cycle. These measurements help predict a patient’s potential response to fertility treatments but are estimates, not an exact count of the entire remaining lifetime supply.